A gem-based plugin whitelist for `safe` mode

I proposed a little while ago to @benbalter, @mojombo, @qrush and @mattr- the possibility of a whitelist of plugins. My thought was to bundle them all as one gem for use with Pages. It would be required in the Gemfile the Pages servers use so there wouldn't need to be any change within Jekyll.

Thinking further, and with the merge of #1557, it makes a lot of sense to allow each of these to be controlled as separate gems, with GitHub's `pages-gem` gem controlling the versions -- that the `pages-gem` would be, itself, the combining element of the whitelist.

(previous derp redacted)