Fix spec generation

[elbrujohalcon]

As described in #7, since #3 mixer is inserting generic specs for every function added.
That effectively prevents compiler warnings when warn_missing_spec is on, but messes up with dialyzer when, for instance, you're mixing in a function that implements a callback defined in a behavior (such situation will lead to warnings like The return type any() in the specification of your_function/1 is not a subtype of another_type(), which is the expected return type for the callback of your behaviour). That's not good.

The best thing to do would be, of course, to import the specs from the source module along with the functions. That might not be possible. If that's so… we should at least make spec additions configurable.