Make the organism and annotation options more general

[lcolladotor]

As a note to myself, in our meeting with @ChristopherWilks and @Nick-Eagles today, the idea of using a text file like the homes_index at https://github.com/LieberInstitute/recount3/blob/master/R/project_homes.R#L44 but for the annotation options came up.

Or well, in general, the goal would be to (a) still provide helpful error messages to users when they use the wrong organism/annotation option/etc and (b) make it so the package can work with custom organisms/annotations that users might have generated themselves that are different from the ones we officially support.