The only exception to the rule is git-flow-support, which has an
explicitly required <base> argument (since we cannot deduce a sane default
name for base).

Furthermore, these <base> arguments are checked to refer to commits on:
- develop (for feature, release)
- master  (for hotfix, support)

Removed any occurrences of optional <base> arguments in finish subactions.
The finishing target branches are clearly defined by the model. The <base>
argument will probably confuse users. If they want the power to merge
those feature branches into *other* branches then develop, for example,
they can still use the magical power of Git itself for that. Gitflow
should not provide such support.