Override Content-Security-Policy header for Asciidoc files

If the server responds with a CSP, then Chrome Extensions will use the
CSP in the header as apposed to the CSP specified in the manifest. This
means for sites like github, we were unable to use inline styles.

To resolve this issue this commit will set the CSP response header on
Asciidoc files to the same value that the Atom editor is using.

Fixes #37
Thanks for this mini-master class on CSP. I also learned a lot...considering I hadn't yet learned much at all about CSP. Now I know something!

> I love asciidoctor and use it daily, so it is my pleasure to be able to give back to the community in any way that I am able. Keep up the great work!

Thanks @rwinch. I really enjoy having you as part of the community! We'll continue to work to find ways for you to love it even more.