Provide a way to give a favored Vega spectrum

[eteq]

The built-in Vega is probably suitable for most imaginable "vegamag" needs, but for some kinds of precision work, some people may care about using a particular vega spectrum (especially given the modern suspicion that it's at least mildly variable).

So it would be useful if there were a programmatic way to provide a favorite Vega. The easiest thing might be to tell users they have to give it as a specutils.Spectrum1D, since then synphot doesn't need to worry about I/O or anything, just that it fits into the Spectrum1D API.

[pllim]

With #243 merged, you can already pass in whatever Spectrum1D you want (even Vega). Does that mean this issue can be closed without further action now?