[COHO] Pull request stats helper

[Shazron <sh...@gmail.com>]

As Steve has said, unfortunately our focus lately has been on platforms,
not plugins.
To help bring to light our PR workload, I worked on a feature to show PR
stats in coho. See:

See:
https://github.com/apache/cordova-coho/pull/134

Typical usage:
coho list-pulls --repo=plugins --stats-only --json

I'll be compiling stats on this every week and putting  it up on a repo in:
https://github.com/cordova/cordova-pull-request-stats

This is for tracking how we are doing in reducing our PR debt so we can
report on it.

For a start, I'll be sending out emails with the current stats on the
plugins and platforms, sorted in descending order of pull request count.