Allow passing in options directly to lib/vulcan.

Greetings,

I'm trying to use vulcanize within the context of a plugin for wintersmith.  Here I've modified the source code to enable this.  I "unstaticized" the options passed in to `vulcan.js` and also made writing to an output file optional.  If the `output` option is not passed into `processDocuments`, no file will be written and the string HTML value will be returned (so it can be used by my wintersmith plugin, which handles file output).  `outputDir` is still used so that paths may be set correctly.  The CLI works the same as before.

Best,
William