Map ':' to command-palette

As a vim user I have the habit of always saving files with `:w`. Currently doing this on vim-mode would edit the buffer.

So mapping the `:` to the `command-palette` avoids this, and mimics the behaviour found in Sublime's vintage package. Although I think that vim-mode should add a different command prompt for this, instead of using the `command-palette`. But that might require Atom core changes and take some time.