Merge spectral regions

[keflavich]

Spectral-regions have been defined in specutils:
astropy/specutils#289

They should be included in regions eventually, especially since CRTF now supports spectral region serialization.

[cdeil]

@keflavich - What does including spectral regions here mean?

Is it just adding something to the I/O and specutils becomes an optional dependency?

Or do you mean changes to the data classes and possibly backwards-incompat breaks?

[keflavich]

It means adding things; I don't see any reason there should be any backward-incompatible breaks.

I'm uncertain whether specutils would become a dependency.