fdoc: treat -spec lines as comments

This makes C-c C-d d (erl-fdoc-describe) display documentation for
functions documented like:

%% foo() is a function.
-spec foo() -> ok.
foo() ->
    ok.
fdoc: use erl_scan to find end of -spec

This handles multi-line type specs properly.
Merge pull request #20 from legoscia/fdoc-spec

fdoc: treat -spec lines as comments