Prior to this commit, the `lein release` task was not
really usable for releases that had qualifiers such
as `-alpha1`, `-beta2`, or `-RC1`.
The reason for this is that the code for parsing the
semver could not handle a qualifier like `-alpha1`
AND a `-SNAPSHOT` qualifier at the same time.
The default `:release-tasks` include a `bump-version`
at the `:release` level, and then the release, and
then a `bump-version` at the `:patch` level. If
you had a version that started out as `1.0.0-alpha1` *or*
`1.0.0-alpha1-SNAPSHOT`, then the `:release` bump
would always set the version to `1.0.0`. Then the
actual release would occur, and then the `:patch` bump
would take you to `1.0.1-SNAPSHOT`.
With this commit, `qualifier` and `snapshot` are
separated into two separate fields in the version map.
This allows us to modify the `bump-version` behavior
so that it works as one would expect with a qualifier
like `alpha`:
```clj
(is (= (bump-version "1.0.0-alpha1-SNAPSHOT" :release)
"1.0.0-alpha1"))
(is (= (bump-version "1.0.0-alpha1")
"1.0.0-alpha2-SNAPSHOT"))
```
This allows the default `:release-tasks` setup to be
used with qualifiers like alpha/beta.
The commit also adds a new `level` that can be passed
to the `change` task: `:qualifier`. If this level
is passed, then the qualifier will be incremented
instead of the major/minor/patch. It is a superset
of the existing alpha/beta/RC behavior but works with
arbitrary qualifier strings.
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