Support username.github.io style GitHub pages

[asmeurer]

There are two kinds of GitHub pages, user/org pages, and project pages. User/org pages are in repos named username.github.io, and are deployed to the master branch. Project pages are in any repo, and are deployed to the gh-pages branch. We currently support the latter. Supporting the former wouldn't be hard, maybe even detecting it automatically.

This is low priority (I don't personally have a need for it).

[gforsyth]

Hey @asmeurer -- we're moving the xonsh docs to live under the xonsh organization and @Carreau has set up a xonsh.github.io style repo to host it. If #54 is updated, then I can add in the github.io support and we can test it out with the xonsh docs (if @Carreau is amenable to this)

[asmeurer]

#54 is next on my todo list for doctr, after 1.1.1, since that is how we deploy docs for SymPy.

[gforsyth]

Implemented in #168