`expect` should list available matchers

[Fuco1]

Alternatively, buttercup-define-matcher could.

The problem I see with this is that people often won't even load buttercup in their emacs where they write the code, so the forms won't evaluate. However, many do and I think discoverability from within emacs is a neat feature to have.

The docstring could be updated dynamically as users define more matchers (buttercup-define-matcher can do the magic).

Related but orthohonal to #35.