Arc desc

[tecknojock]

fixes https://github.com/xiagu/jekyll-rp_logs/issues/21

and adds descriptions to arcs.

[irisfofs]

A couple things:

• I do think having descriptions for arcs is a good feature, but they need to be specified in only one place, which means probably something similar to the _tags.yml file.
• This is especially important because RPs can belong to multiple arcs, so the arc_description field will do super wacky things in that scenario.

Reorganization of the Arcs page is something I have some other ideas for that I hope to actually address sooner rather than later.