Add link to Emacs documentation for shell-command.
Add shell_command_on_region which will always check for a region.

This is simply to stay in sync with the Emacs code.
Use the async version of `set_timeout`.
[#2] Create thread for shell commands, to avoid blocking Sublime.

I thought that using sublime.set_timeout() would avoid blocking, but it doesn't.

Fixes #2.
Bump version number and update changelog.
Merge branch 'release/0.3.0' into develop