Do not open a (migration-)file if escape key was pressed

I've "stolen" this from [SublimeText2RailsRelatedFiles](https://github.com/luqman/SublimeText2RailsRelatedFiles/blob/master/Rails.py#L228) as it was sort of annoying.

Be aware that it worked for me but I absolutely have no clue about python in general. I was a bit confused though as everything was indented with 2 spaces except the method I'd edited.