#1 Extension to use data: URL

[Niko Matsakis]

The extension as written leaves files in the $mediawiki/images/graphviz
directory which can not be safely deleted, and which simply grow over
time.

This patch adds an option to generate a data: URL instead; this has the
downside that the graph must be re-rendered every time, and for large
graphs results in a lot of data being inserted into the page. However, it
has the upside that no files are left on the file system --- the mediawiki
cache prevents excess rendering from occurring for small wikis like mine.

A simple improvement would be to use the cache only for small files: of
course, for small files it hardly matters anyhow.

[Niko Matsakis]

patch to use data:// URLs