Support publishing to personal pages sites in quickstart

Previously pelican-quickstart would assume that the site it created for GitHub
Pages should be published to the gh-pages branch. This is correct for project
pages, but not correct for personal pages. Personal pages, which live in a
user's special username.github.io repository, are instead deployed to the
master branch. This means that if you did pelican-quickstart and tried to
publish your new personal site with make github you'd see nothing (or whatever
old pages site you had floating around in master).

ghp-import already supports publishing to different branches, so publishing
to the correct branch is just a matter of correct configuration and updating
the Makefile to pass the branch along to ghp-import. pelican-quickstart now
asks if the user wants to publish to GitHub Pages, and if so, asks if this
is a personal page and chooses the correct branch appropriately. I preferred
this approach to prompting for an arbitrary branch because I felt that
choosing the branch would feel more intimidating to someone using
pelican-quickstart for the first time.

This essentially ports changes I made to my personal pages site at
jculpon@82cae477a9e8712b90654f6432464369ebcc7ae5
Merge pull request #1181 from jculpon/ghpages-config

Support publishing to personal GitHub Pages sites in quickstart