ENH: add --negate_sample_id_fp to filter_samples_from_otu_table.py

@ebolyen and I pair-programmed this.

Fixes #1117.
@jairideout, @ebolyen and I just discussed. 

We should update the name ``samples_to_keep.txt`` to ``ids.txt``, and then change this and the next description to be:

*Keep samples where the id is listed in ids.txt.*
and
*Discard samples where the id is listed in ids.txt.*