Document pants.ini configuration options.

Mostly a repost of https://rbcommons.com/s/twitter/r/67/ post move to
pantsbuild.

Add a simple system to register pants.ini options so we can generate a
configuration reference page on our website. Lots of required, undocumented
keys in pants.ini has been identified as an issue for new users and this
change lays the groundwork. Once the pattern has been defined I'll move all
options over to this style in a few future reviews.

Similar to command-line flags, pants developers explicitly define options
rather than fetching keys from the config directly. We register these keys
enabling the generation of a docs site. A few examples are provided here, and
we'll do the rest after reaching consensus on the format. The goal is
generating documentation for our users, so we explicitly avoid going overboard
and rewriting the whole config system (e.g.: unifying pants.ini & command-line
flags which is out of scope for this change).

Eliminate two options that are not obviosuly desiarable to allow users to
tweak. In 67 we agreed to remove these rather than document as tuneable:
tasks.build_invalidator, reporting.reports_dir

Reviewed at https://rbcommons.com/s/twitter/r/183/