Dunamai

* Added the `--bump` command line option and the `bump` argument to
`Version.serialize()`.
* Fixed an issue with Git annotated tag sorting. When there was a newer
annotated tag A on an older commit and an older annotated tag B on a
newer commit, Dunamai would choose tag A, but will now correctly choose
tag B because the commit is newer.
* With Git, trigger the dirty flag when there are untracked files.
([Contributed by jpc4242](https://github.com/mtkennerly/dunamai/pull/6))

* Fixed ambiguous reference error when using Git if a tag and branch name
were identical.

* Previously, when there were not yet any version-like tags, the distance would
be set to 0, so the only differentiator was the commit ID. Now, the distance
will be set to the number of commits so far. For example:

* No commits: base = 0.0.0, distance = 0
* 1 commit, no tags: base = 0.0.0, distance = 1
* 10 commits, no tags: base = 0.0.0, distance = 10

* Added `--debug` flag.

* Added these functions to the public API:
* `serialize_pep440`
* `serialize_semver`
* `serialize_pvp`
* `bump_version`

* Changed the `Version` class to align with Dunamai's own semantics instead of
PEP 440's semantics.

Previously, `Version` implemented all of PEP 440's features, like epochs and
dev releases, even though Dunamai itself did not use epochs (unless you
created your own `Version` instance with one and serialized it) and always
set dev to 0 in the `from_git`/etc methods. The `serialize` method then
tried to generalize those PEP 440 concepts to other versioning schemes,
as in `0.1.0-epoch.1` for Semantic Versioning, even though that doesn't
have an equivalent meaning in that scheme.

Now, the `Version` class implements the semantics used by Dunamai, giving
it more power in the serialization to map those concepts in an appropriate
way for each scheme. For example, `dev0` is now only added for PEP 440 (in
order to be compatible with Pip's `--pre` flag), but `dev.0` is no longer
added for Semantic Versioning because it served no purpose there.

API changes:

* `post` has been renamed to `distance`, and its type is simply `int`
rather than `Optional[int]`
* `epoch` and `dev` have been removed
* `pre_type` has been renamed to `stage`
* `pre_number` has been renamed to `revision`, and it is no longer required
when specifying a stage
* Improved error reporting when the version control system cannot be detected
and when a specified VCS is unavailable.
* Improved the default regular expression for tags:
* It now requires a full match of the tag.
* It now recognizes when the `base` and `stage` are separated by a hyphen.
* It now recognizes when the `stage` and `revision` are separated by a dot.
* It now allows a `stage` without a `revision`.