add ghpRoot attribute handling

[paddelkraft]

creating a project site it will automatically be placed in a subfolder like

paddelkraft.github.io/projname

being able to run docpad with the same offset http://localhost:9778/projname/ and mark subdirectory projname as the root for github pages like this

outPath: 'out' # default
ghpRoot: 'in5'

makes it possible to use the same internal links in md files and layouts without needing do do any templating which is really nice.

[paddelkraft]

ghpRoot: 'projname'

[balupton]

I have become too preoccupied by other things to review, merge, and release this PR.

However, you should have received an invite a while ago to join the DocPad Extras Team, which will give you write access to this repository, so you can merge in the PR, and become one of the maintainers of this repository.

For an orientation, or if you need any assistance following the semi-automatic release process from CONTRIBUTING.md, reach out via https://bevry.me/discord/dev