More thorough/advanced line-flux formulation

This issue builds off of discussions in #235, #314, and #324.

To summarize: #324 implemented a "quick and dirty" way of doing line flux measurements - basically it only considers whole-pixels. The "right" way is to also include the fractions of pixels that are on the edges. For a more in-depth discussion of this with math and everything see
https://github.com/astropy/specutils/issues/314#issuecomment-421109230 .

This issue is to improve the `line_flux` function to account for the fractional edge pixel. I think the linked comment above lays that out reasonably thoroughly, but if more clarification is needed/changes proposed, we can hash it out in this issue.

By reference this will also affect `equivalent_width`, `SNR`, etc., I think which is a good thing.

Start with the line_flux function and read the linked discussion in issue #314, along with the context from issues #235 and #324. The work is complete when line_flux accounts for fractional edge pixels and the related equivalent_width and SNR behavior is considered.

Written by the indexing model from the issue text.