Additional theme options

Now that we've powered up themes to have substantive logic of their own, I think they're probably going to want options of their own too. Example use cases: more specialized filtering as in #274; passing in a prelude/introduction file; setting a baseurl (annoying but sometimes needed).

How about just including a `themeOptions` object nested within the formatter options that we [already send](https://github.com/documentationjs/documentation/blob/master/lib/commands/build.js#L48-L51)?

Then the node api could accept `themeOptions` in its options, and the cli could accept a browserify-like [subargs syntax](https://github.com/substack/subarg) or something like `--theme-XXX value --theme-YYY value2`.

cc @tmcw @jfirebaugh