Add support for intensities in erg/cm^2/s in unit equivalencies

I would like to easily be able to convert between Fnu, Flambda, and nuFnu (same as lambdaFlambda). Technically speaking, nuFnu is not a spectral density, but it *would* be nice to do:

```
>>> (u.erg / u.cm**2 / u.s).to('mJy', 1, u.sd('microns', 40))
```

since this is unambiguous. Maybe we should just have one equivalency called 'spectral' which encompasses the current ``sp``, ``sd``, and also nu*Fnu or something like that? I don't think there is any ambiguity between the equivalencies in ``sp`` and ``sd``, right?

EDIT: fixed code above