Implement backwards interactive search

Adds commands:
 buffer-search-backward
 buffer-repeat-search-backward
 buffer-repeat-search-forward

Implements ustring\urfind(s, text [, init])
Improve backward-search implementation and update docs.
Move new changelogs items to correct place
Merge branch 'shalabhc/backward-search'

Conflicts merged:
	Changelog.md
	lib/howl/commands/edit_commands.moon