Add option to emit a comprehensive index of the namespace metadata

[tobias]

This makes it easier to generate a single autodoc/ tree in a multiproject maven build, since the existing index.clj doesn't contain enough data to feed back to autodoc.build-html/make-all-pages.

Example usage: we use this in Immutant (http://immutant.org/) to build docs for each module, then discard all but the raw indexes, which are fed to:
https://github.com/immutant/immutant/blob/master/docs/src/main/clojure/immutant/docs/autodoc.clj

If there is a better way to solve the multiproject doc problem, I'm certainly open to that.

[tomfaulhaber]

Tobias,

This is a good addition. I'd certainly be interested in having this documented for others to use as well and if there's a way to include support for the second stage of generating the multi-project docs directly into autodoc (or a supporting project as I do with autodoc-index), that might be a great idea.

[tobias]

@tomfaulhaber - that's a good idea, I'll take a look at implementing the second stage in the next couple of weeks, and also document this behavior.