Unify reddening_* functions; add extinction(); docs

This commit does a few things:

* Unify reddening_fm() and reddening_ccm() functions into a single reddening()
  function with a keyword argument for different models. Currently there are
  five different models.
* Add an extinction() function that is the same is reddening, but in magnitude
  space. This is useful for testing and plotting because the original
  extinction laws are given in magnitude space.
* Expand and reorganize docs for the extinction() function, including
  plots comparing the extinction functions.