New feature: allow merge_taxa to keep recored of merged (and inextension tax_glom).

[derele]

Hi Paul,

are you interested in adding a (tiny) new feature to phyloseq?

I needed something to track what tax_glom and merge_taxa are doing and implemented a small feature to allow for a function (mostly relevant for pasting together or counting) to be applied to the length>1 entries of the merged/agglomerated taxa.

I exposed the argument I implemented for merge_taxa only to tax_glom, not to any other of the relevant higher level functions (the only other relevant one would probably be tip_glom, I guess).

I left the default intact, so it should not break anything. I added the required documentation (via devtools::document()) and did my best to make sure that devtools::check() complains only about stuff that has nothing to do with my changes.

Feel free to pull or ignore!

All the best, Emanuel