On OS X, beet config -e doesn't work for initial setup by default

[sampsyo]

By default (when $EDITOR is not set), the beet config -e command will try to use OS X's open command to edit the config file on that platform. Unfortunately, that command doesn't work when the file does not exist—when setting up beets for the first time, for example.

We may need to run touch first to create the file, and then use open -t to get the default text editor.

Background in a mailing-list thread:
https://groups.google.com/d/topic/beets-users/FDa_3fO3sBM/discussion

[tomjaspers]

I didn't mention it in the commit or in the code, but this also creates any directories needed (which was the case in the mailing-list thread).