Allow users to specify a message directly when performing a release

[thanethomson]

Just as in #15, we should be able to perform releases automatically directly from the CLI.

At present, a release triggers the opening of a user's editor:

> unclog release --version v0.1.2

We should facilitate generating a release directly from the CLI:

# Doesn't open the editor
> unclog release --version v0.1.2 --message "Summary of the release changes"

# Shortened
> unclog release -v v0.1.2 -m "Summary of the release changes"

[rnbguy]

Or a file containing the release summary, similar to gh or hub.

> unclog release --version v0.1.2 --message-file release-sumary.txt
# shortened
> unclog release --version v0.1.2 -F release-summary.txt