They're only needed when generating docs, so no need to load them all
the time. This also lets us more easily test situations where the Maruku
filter is not defined or its dependencies are unavailable.
I recently added a rescue from LoadError if the Maruku and Textile
filters from haml-contrib aren't available. Since we don't test those
filters here, there's no longer any need to depend on haml-contrib even
for development.
See issue #551
[ci skip]
