Update plugin to work with Sublime Text 3.

This will make this plugin work in Sublime Text 3, while maintaining compatibility with ST2. Changes are as follows:

* All `print` statements are changed to `print()` functions.
* The import method for `slug.py` from `slug_plugin.py` is changed to work in both ST2 and ST3.
* The text replacement after separator input is moved to a separate command, to fix an error in ST3.

I've tested it a bit in both Sublime Text 2 and 3, but some more testing would be nice before accepting this patch and pushing a new version to Package Control and such.