This was prompted by an issue over at MELPA, where they were having
trouble packaging stable versions of erlang-mode due to Erlang’s odd
version-strings: https://github.com/milkypostman/melpa/issues/2553.  So
with this patch, 'OTP-18.0.5' is valid and parsed as '(18 0 5).

Other changes in behavior:

- “.” can be used as a priority separator.  This seemed appropriate
   since Emacs allows changing of `version-separator' to strings other
   than “.”, and in such cases “.” might be used to mark priority.

- The docstring said “.5” was invalid, when it actually was.  I’ve made it
   explicitly valid, and added tests for it.

- The docstring said “22.8X3” was invalid, when it actually was; it got
   parsed as '(22 8 24 3).  I’ve made it really invalid.

- I’ve made strings like “alpha3.2” valid, and parsed as '(3 2 -3).