Expand the use of refgroups

[mhagger]

Reference groups or "refgroups" were introduced in https://github.com/github/git-sizer/pull/84 as a way to limit the references that git-sizer uses as starting points when scanning repository contents. This PR expands on that feature in the following ways:

• Allow refgroups to have user-specified names. For example,
• Allow refgroups to be nested. Nested refgroups are only matched against references that also match their parent(s).
• Allow refgroups to have user-specified names.
• Allow refgroups to be used as --include=@REFGROUP and --exclude=@REFGROUP option arguments.
• Have git-sizer emit a count of references in each defined refgroup.
• Add some more pre-defined refgroups: pulls (refs/pull/**) for GitHub-style pull request references, and changes (refs/changes/**) for Gerrit-style changeset references.

Putting this all together, one could use something like this config:

[refgroup "tags.releases"]
        name = Release tags
        includeRegexp = refs/tags/v\\d+\\.\\d+(\\.\\d+)?

and run git-sizer the following way to see counts of branches and tags, the latter broken down by release tags and other tags:

$ git-sizer -v --include=@branches --include=@tags
| Name                         | Value     | Level of concern               |
| ---------------------------- | --------- | ------------------------------ |
[…]
| * References                 |           |                                |
|   * Count                    |    18     |                                |
|     * Branches               |     5     |                                |
|     * Tags                   |     6     |                                |
|       * Release tags         |     5     |                                |
|       * Other                |     1     |                                |
|     * Ignored                |     7     |                                |
|                              |           |                                |
[…]

Any references that are excluded from the scan entirely are tallied under "Ignored".

I also made the --include and --exclude options more flexible. Aside from the existing --include=PREFIX style, one can now also use --include=/REGEXP/ as a shorter version of --include-regexp=REGEXP and --include=@REFGROUP as an equivalent of --refgroup=REFGROUP. The old-style versions of these options have been deprecated.

This PR is written so that it can be read commit-by-commit. It sprouts from the branch for https://github.com/github/git-sizer/pull/86 because it uses some of the new test features introduced by that PR.

/cc @ttaylorr and @tgummerer as possible reviewers.

[mhagger]

@tgummerer: Commit d508da2473307df13addd7a0d6c14bdd58bd433f should fix the problem that I mentioned here.