fit_continuum: implement continuum fitting windows

The 'window' and 'weights' parameter in specutils.fit_continuum should be implemented to allow users to indicate which spectral regions to be used in fitting, and what weights should be given to these regions.

Start at the specutils.fit_continuum entry point and inspect how the window and weights parameters are currently handled. Implement behavior allowing callers to select spectral regions and assign their fitting weights, then verify that continuum fits reflect those selections and weights.

Written by the indexing model from the issue text.