Merge pull request #1448 from Kazark/master

Implement :execute command
Bind gf in Visual mode to go to file in selection
Bind <C-w>gf in Visual mode to open file in window

I could not find precedent for how to unit test this, so I did not add a
unit test. :s
Get the selected text as the filename to open (gf)
Plug in visual <C-w>gf to open new window