Added 'Delete' as a shortcut key for deleting the selected gist

[GabrielNicolasAvellaneda]

@hackjutsu Adding Del key as shortcut comes in handy when you want to quickly cleanup your gists. The code on app/index.js could be refactored to remove duplication from older code, but I want to start easy for now. I will push some code refactoring later in another commit if you allow me to do this. I also have more improvements to commit. Hope this helps. ;) When you have some time can you please take a look at this. Thanks!

[hackjutsu]

Thanks for the contribution! @GabrielNicolasAvellaneda