Have a more general way to recognize models for fitting that have positional parameters

From #210 - see https://github.com/astropy/specutils/pull/210#discussion_r214480177 for more detail.

The basic gist: models like polynomials can't be "copied" trivially because you need a way to know how many parameters they have. `inspect` gives everything needed to recognize this, and I think I solved it in the linked comment... but it needs a PR with proper testing and everything, as it's pretty complex.