Support more flexibility for PDF's

Didn't spend much time on this, but this adds support for a passed in options parameter as well as a ret callback transformer callback. Currently only used with pdf's but can be easily extended to other filetypes as well.

I use it to add a custom page renderer for your fantastic pdf-parse library (thank you!)

crawler(url, undefined, {pdf: {pagerender: customRenderer, callback: transformReturn, ...}})

Not fleshed out, but if you wish to implement or improve upon it I found it very useful.