Make insert content work in Sublime 3

`begin_edit()` and `end_edit()` are restricted in Sublime 3, so we need to use
the `view.run_command()` method to access the `edit` instance. Vaguely described
in the Sublime 3 porting guide:

    http://www.sublimetext.com/docs/3/porting_guide.html
Merge pull request #1 from nifty/master

Make insert content work in Sublime 3