Lua API: access and set all available options

the first point of this commit is to allow all options to be read from
lua. this has a number of uses for plugin writers. they are grouped into
a couple of tables depending on what they control:
`vis.options`: table with global configuration
`win.options`: table with window specific configuration

the second point is to allow you to set all these options as if they
were simply lua variables. technically this is already possible by
using `vis:command("set ...")` but personally I think this interface
is cleaner. Note that this already possible for some things like the
current mode (eg. vis.mode = vis.modes.VISUAL). examples:
`vis.options.ai = true`
`vis.options.brk = " !?."`

there are a number of related issues and pull requests:
closes #803: Lua API: let plugins read the values of options
closes #812: Window layout property
supersedes/closes #717: Add ability to access tabwidth from Lua
supersedes/closes #1066: expose UI layout and allow it to be set from lua API

ui-terminal.c

@@ -672,6 +672,11 @@ static bool ui_init(Ui *ui, Vis *vis) {
 	return false;
 }
 
+enum UiLayout ui_layout_get(Ui *ui) {
+	UiTerm *tui = (UiTerm *)ui;
+	return tui->layout;
+}
+
 Ui *ui_term_new(void) {
 	size_t styles_size = UI_STYLE_MAX * sizeof(CellStyle);
 	CellStyle *styles = calloc(1, styles_size);

ui.h

@@ -116,5 +116,6 @@ struct UiWin {
 };
 
 bool is_default_color(CellColor c);
+enum UiLayout ui_layout_get(Ui *ui);
 
 #endif

view.c

@@ -910,6 +910,10 @@ void view_wrapcolumn_set(View *view, int col) {
 		view->wrapcolumn = col;
 }
 
+int view_wrapcolumn_get(View *view) {
+	return view->wrapcolumn;
+}
+
 bool view_breakat_set(View *view, const char *breakat) {
 	char *copy = strdup(breakat);
 	if (!copy)
@@ -919,6 +923,10 @@ bool view_breakat_set(View *view, const char *breakat) {
 	return true;
 }
 
+const char *view_breakat_get(View *view) {
+	return view->breakat;
+}
+
 size_t view_screenline_goto(View *view, int n) {
 	size_t pos = view->start;
 	for (Line *line = view->topline; --n > 0 && line != view->lastline; line = line->next)

view.h

@@ -359,7 +359,9 @@ enum UiOption view_options_get(View*);
 void view_colorcolumn_set(View*, int col);
 int view_colorcolumn_get(View*);
 void view_wrapcolumn_set(View*, int col);
+int view_wrapcolumn_get(View*);
 bool view_breakat_set(View*, const char *breakat);
+const char *view_breakat_get(View*);
 
 /** Set how many spaces are used to display a tab `\t` character. */
 void view_tabwidth_set(View*, int tabwidth);

vis-cmds.c

@@ -129,14 +129,24 @@ static void windows_arrange(Vis *vis, enum UiLayout layout) {
 	vis->ui->arrange(vis->ui, layout);
 }
 
-static void tabwidth_set(Vis *vis, int tabwidth) {
+void vis_tabwidth_set(Vis *vis, int tabwidth) {
 	if (tabwidth < 1 || tabwidth > 8)
 		return;
 	for (Win *win = vis->windows; win; win = win->next)
 		view_tabwidth_set(win->view, tabwidth);
 	vis->tabwidth = tabwidth;
 }
 
+void vis_shell_set(Vis *vis, const char *new_shell) {
+	char *shell =  strdup(new_shell);
+	if (!shell) {
+		vis_info_show(vis, "Failed to change shell");
+	} else {
+		free(vis->shell);
+		vis->shell = shell;
+	}
+}
+
 /* parse human-readable boolean value in s. If successful, store the result in
  * outval and return true. Else return false and leave outval alone. */
 static bool parse_bool(const char *s, bool *outval) {
@@ -241,16 +251,8 @@ static bool cmd_set(Vis *vis, Win *win, Command *cmd, const char *argv[], Select
 
 	switch (opt_index) {
 	case OPTION_SHELL:
-	{
-		char *shell = strdup(arg.s);
-		if (!shell) {
-			vis_info_show(vis, "Failed to change shell");
-			return false;
-		}
-		free(vis->shell);
-		vis->shell = shell;
+		vis_shell_set(vis, arg.s);
 		break;
-	}
 	case OPTION_ESCDELAY:
 	{
 		TermKey *termkey = vis->ui->termkey_get(vis->ui);
@@ -264,7 +266,7 @@ static bool cmd_set(Vis *vis, Win *win, Command *cmd, const char *argv[], Select
 		vis->autoindent = toggle ? !vis->autoindent : arg.b;
 		break;
 	case OPTION_TABWIDTH:
-		tabwidth_set(vis, arg.i);
+		vis_tabwidth_set(vis, arg.i);
 		break;
 	case OPTION_SHOW_SPACES:
 	case OPTION_SHOW_TABS: