waylan Add hljs_style config to mkdocs theme.
This is primarily to allow third party themes which inherit from the 
mkdocs theme (such as the bootswatch themes) to alter the code 
highlighting style. Previously all of the themes hosted their own 
highlighting styles, but now that we are using the CDN, it is easier to 
use a setting, which downstream themes can override.
Latest commit 06608e9 Jul 17, 2018