Crash when target file removed

[osjupiter]

note crashes when target file removed or renamed.

This problem causes a error with vim configued set writebackup.
Because vim with set writebackup rename the old file and create new one on save.

crash log

go run  . --no-editor aaa 
2019/04/09 22:13:32 [c.Args():  [aaa]]
2019/04/09 22:13:32 [Call openURL]
2019/04/09 22:13:32 [Call http.ListenAndServe]
2019/04/09 22:13:32 [Call runEditor()]
2019/04/09 22:14:01 [event: "aaa": RENAME] # save on vim or `mv aaa bbb`
2019/04/09 22:14:01 [open aaa: no such file or directory]
panic: open aaa: no such file or directory

goroutine 20 [running]:
main.markdownFileToHTML(0x7fff6e130336, 0x3, 0x2, 0xc00009b0a0, 0x201)
	/home/ooken/siken/note/handler.go:137 +0x245
main.(*markupHandler).Start(0xc0000b4cd0)
	/home/ooken/siken/note/handler.go:78 +0x4eb
created by main.run
	/home/ooken/siken/note/main.go:51 +0x21a
exit status 2

I was troubled with this problem so I wrote this for others.

version 0.3.0