Add new prerelease options

[Martín Acosta]

Command: chan release <semver>

Chan is going to ignore the prerelease (like v2.0.0-beta1) by default.

We are going to have two new options:

• --prerelease is going to allow prerelease versions.
• --merge-prerelease is going to allow prereleases and merge them on the next official release.

[Martín Acosta]

closed by: #35