Add support for scripting

[hal7df]

Add the ability to run commands from a script file (much like bash scripts).

iash scripts must have one of the following two lines at the start of the file:

• #!iash
• #!iash@appname

The first case, a generic script, will be accepted by any program that uses iash, although in the future this should be disabled by default (allowing the client to reenable it if they wish). The second case is a specific script, which iash will only run if appname is the currently configured application name.

Comments should have the same syntax as bash, with a # character commenting an entire line (this should also be supported in the interactive shell as well)