Allow to link sources across several git servers

[tristanlatr]

pydoctor supports generating documentation covering more than one package/project,
in which case it is not certain that all of the source is even viewable below a single URL.
We currently ignore this limitation by not assigning sourceHref, but it would be good to add support for it.

A way to do it would be to match (sub)package names to different --html-viewsource-base and --project-base-dir directories as well as potentially covering multiple git server kinds (option --html-viewsource-template).

Pydoctor would then be called like this to use the new feature:

pydoctor
        --html-viewsource-base=zope.interface:https://github.com/zopefoundation/zope.interface/tree/master
        --html-viewsource-base=zope.component:https://github.com/zopefoundation/zope.component/tree/master
        --html-viewsource-base=someotherpack:https://bitbucket.org/lalinsky/python-phoenixdb/src/master
        --project-base-dir=zope.interface:./zope.interface/
        --project-base-dir=zope.component:./zope.component/
        --project-base-dir=someotherpack:./someotherpack/
        ./zope.interface/src/zope ./zope.component/src/zope ./someotherpack/someotherpack

This also open-up the question of whether should allow different values per package for options --docformat, and potentially other options as well.