Add support for post descriptions

[joshuaboniface]

Adds support for post descriptions, i.e. subtitles. These are presented in an italicized h4 header under the post title if present in the Hugo post configuration.

[grinsted]

Another approach would be to include a new post_header.html partial implemented in a similar way to #126 . That would be a flexible approach and would allow people to come up with their own code for what should be in the header.

[joshuaboniface]

I ended up moving to another theme for my site, so I probably won't touch this any further. But feel free to run with it if you want, or close this out :-)