add emacs to scoop-extras bucket

As discussed in lukesampson/scoop#30.

Note: I kept the other 2 executables:
   - I almost always use runemacs from the command line, almost never emacs.exe, to get an emacs server going without blocking the console.
   - I usually use emacsclientw to connect to an emacs server.

Since this is in extras, I thought it would be OK to have shims for all three. Let me know if this is not cool. Thanks!