Decide on whether SpectralRegion.extract_from or similar should exist

[eteq]

Most of the context is here: https://github.com/astropy/specutils/pull/289#issuecomment-419257644 - for 0.4 we decided to implement the `extract_region function, but we should decide if the public API should be the method-style or the function-style.

My vote is for region.extract_from(spectrum), as I think that's easier to work with and gives the possibility of specialized regions that do specialized forms of extraction (a functional approach doesn't allow that because there's only one function), among other reasons. Other thoughts?

[nmearl]

@eteq is this in addition to the function-style extract_region we already have, or is this ticket no longer relevant?