Rebuild theme: Add config for GitInfo.AuthorDate support

[leslitagordita]

This is the theme rebuild for Hercules PR: https://github.com/linode/hercules/pull/118

[nmelehan]

Testing and updates:

• I created a new staging server to test that the standard gulp publish flow still works and doesn't trip over the .GitInfo object. It works normally; for example, I removed the modified date from the front matter of the Getting Started guide (on a test branch). The staging server publishes normally, and the guide shows today's date in the Updated label:
  http://45.33.75.101/docs/getting-started/

• I updated the tests to make modified not required, and then updated the Writer's Formatting Guide and the YAML documentation in the Hercules repo accordingly. I then tested that Travis still worked when the modified tag was absent from the front matter of a guide:
  [DO NOT MERGE] Last modified date config test missing modified #2288

• I observed that when running the gulp dev command, the modified date from Git isn't used. As best I can tell, this is because gulp dev runs the hugo server from the hercules directory, with the content directory set to ../docs/docs. If the modified date is still present on a guide, gulp dev will fall back to displaying that. If modified is missing, then gulp dev will show the original publish date in the Updated label on a guide.

  I tried to fix this, but I was unsuccessful: https://github.com/nmelehan/hercules/commit/6c846372645e2e1b8f669e2473cbe9987eea0ab8

  The idea is to set the --source directory to be the docs directory. This is what the gulp publish command does, and it works as expected (in that the .GitInfo object is accessible). In addition to setting the --source directory, I use the --themesDir, --theme, and --config options to specify the themes directory, theme name, and location for config.toml so that they refer back to the hercules directory. This part doesn't work; when hugo builds, it says it can't find various shortcodes.

  I tried also specifying the --layoutsDir option, which succeeded in getting things to build, but the CSS/javascript/all styling assets were missing from the built site. I abandoned working on this, as accepting that gulp dev will show an inaccurate date is probably an acceptable compromise for now.