Add basename support for crawl mode

[alexlande]

👋

I tried out the new crawl mode today, and it's awesome!

My app uses basename with React Router to handle deployment to GitHub pages, so my routes look like /about, but the deployed URLs look like /project-name/docs/about. This broke the crawler, understandably. It generated the pages but put all of them after the index in a nested subdirectory that matched the basename.

This change lets you pass in a basename so that the crawler will disregard it, and all of the pages get built correctly. If you're interested in something like this let me know and I can add some tests.

[coveralls]

Coverage decreased (-0.9%) to 99.107% when pulling 7a6e592 on alexlande:feature-basenameCrawling into 09001fe on markdalgleish:master.