Add --show-only-field option to app-info command

For scripting purposes, it would be nice to get just a single piece of data in an easy, machine-readable way.

Examples:

    $ tsuru app-info --show-only-field=application
    tsuru-dashboard

    $ tsuru app-info --show-only-field=repository
    git@tsuru.corp.surveymonkey.com:tsuru-dashboard.git

    $ tsuru app-info --show-only-field=address
    tsuru-dashboard.10.128.40.70.xip.io

Cc: @fsouza, @jblancett