Parallelize plyr-dependent functions

[Vladimir Mikryukov]

It should be quite easy to parallelize plyr-dependent functions to speed up processing of data.

Functions that could be modified to accept parallel option:

• phyloseq_mult_raref
• phyloseq_sep_tax
• parse_silva_tax_batch
• parse_taxonomy_amptk_batch
• parse_amplicon_table - just add ... as argument to silva_tax_parse_batch

[Vladimir Mikryukov]

Finally, the first stepping stone!
Multiple rarefaction was parallelized with plyr (decf7ae).