Redefining keymaps to avoid conflicts with default Atom keymaps

See #86.

I used [the defaults from the Sublime Text plugin](https://github.com/sergeche/emmet-sublime/blob/master/misc/generate-keymap.py) for Linux/Windows and then added the Darwin shortcuts we already had here under a more specific selector.

I also did a little normalization with the definitions and made the shortcut list in the readme into a table with a new Linux/Windows column.

This should cover us non-Darwin users while keeping the shortcuts the same for those who are already used to them (I did not bring in the OSX shortcuts from the Sublime plugin). However, I do not have a Mac at my disposal to test that assertion, so please take a good close look and let me know if you think something needs to be changed. I'll be happy to squash in any fixes or better ideas you might have.