Add script for generating web-embeddable help

Mallard's generated HTML help looks great on its own, but it's not
really directly embeddable into another website without some
modifications. Rather than write a new XSLT transformation (...) this
commit adds a simple script that pulls out the requisite bits of HTML,
and perform some CSS munging to get the rules to fit into Meld's
existing web site without too much weirdness.

In order to run, this script requires Python's BeautifulSoup 4 module,
and a command-line executable copy of Sass.

There's a reason I used Sass for this at the time, but now I can't for
the life of me remember why.
Rework VC plugin loading to use importlib, requiring Python 2.7
.gitignore: Add build/ folder
vcview, vc.git: Identify and don't descend ignored folders (bgo#684400)