Override default keybinding for cmd-enter

[kylebarron]

I thought it was weird that you'd need to manually add something to your keymap.cson to use cmd-enter, because I use the Hydrogen package often and I never had to edit the keymap to use cmd-enter with that package.

Looking at Hydrogen's keymap.cson, the included changes should force the use of cmd-enter for this package. I've also added [data-grammar="source r"] because it's likely anyone using this package would only be using it for files covered by the R grammar file, but you could take that out if you want.

I believe this could fix #52 and #51.

[pimentel]

fantastic. thanks, @kylebarron! going to test this out and merge later this week.

[kylebarron]

Also, thanks for the awesome package! It was really easy to port all the functionality to running code in Stata.