Support pre-release versions

[alexgig]

Add support for --alpha, --beta, and --rc flags when using grove bump.

Ref:

• http://semver.org/#spec-item-9
• http://semver.org/#spec-item-11

[alexgig]

Elm compiler does not currently support pre-release versions so this is not possible at the moment.

[cscalfani]

The best way to have someone else test a package prior to its release, at the moment, is to push to a separate branch, i.e. next, then have them clone/pull the repo, checkout that branch and use the --link option on grove install to install in locally. Don't forget to properly set the grove-links.json file.