Adds multiple description field support

maker.py: Adds stub function to grab descriptions in hopes
of later being able to add multiple description field support
to the interactive overlay creation session.

X-Gentoo-Bug: 465730
X-Gentoo-Bug-URL: https://bugs.gentoo.org/465730
overlay.py: Removes old branch tag components

In favor of setting the branch variable in each source tag, the old
branch tag components have been removed. Proper feeds support has
also been added for overlay dictionaries.

subpath-2.xml: Alters test to reflect new branch setting method.
maker.py: Adds autofill support for select mirrors

github.com, git.overlays.gentoo.org, and bitbucket.org
mirrors will have their extra source URLs, homepage, and
feeds filled out for them automatically.
maker.py: Adds Exception raising for errors
maker.py: Migrates prints to self.output messages

Also cleans up large messages to tidy up the code and make it more
legible.
utils.py: Adds get_input() and get_ans() functions

maker.py: Migrates to get_ans & get_input from utils.py