feat(contrib): add bumpver tool to help with semantic version releases

Adds a simple go command-line tool `bumpver` that will help us update source and docs with semantic version numbers as described in its README.md. Skip to the bottom to ignore the vendored code and read the important stuff.

TESTING: You can follow the instructions in README.md and simply paste those commands in, then `git diff` the result (and `git stash` afterward):

```console
$ make -C contrib/bumpver/ test build
$ ./contrib/bumpver/bumpver -h
Updates the current semantic version number to a new one in the specified
source and doc files.

Usage:
  bumpver [-f <current>] <version> <files>...

Options:
  -f --from=<current>  An explicit version string to replace. Otherwise, use
                       the first semantic version found in the first file.
 
$ # update from an explicit (bad) version string
$ ./contrib/bumpver/bumpver -f latest 0.13.3 contrib/coreos/user-data
$ ./contrib/bumpver/bumpver -f 0.13.0-dev 0.13.3 \
    version/version.go \
    client/deis.py \
    client/README.rst \
    client/setup.py \
    controller/deis/__init__.py \
    docs/installing_deis/register-admin-user.rst \
    docs/using_deis/install-client.rst
$ # update from the first semver string found
$ # this type of command should now be enough to bump everything
$ ./contrib/bumpver/bumpver 0.14.0 \
    version/version.go \
    client/deis.py \
    client/README.rst \
    client/setup.py \
    contrib/coreos/user-data \
    controller/deis/__init__.py \
    docs/installing_deis/register-admin-user.rst \
    docs/using_deis/install-client.rst
```

Closes #2002.

[skip ci]
Good catch--I looked around briefly for something like that but missed it. We don't really need to compare versions or any of `go-semver`'s other functionality, and they didn't use `regexp`, the right tool for the job. :)