Decide on whether SpectralRegion.extract_from or similar should exist

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?

Read the context in pull request #289, especially the referenced issue comment, and compare the existing extract_region function with the proposed SpectralRegion.extract_from(spectrum) method style. The work is done when the project has reached and recorded a decision about the public API.

Written by the indexing model from the issue text.