ENH: add support for the different versions of uchime_denovo within vsearch

[mikerobeson]

Somewhat an extension of #75, but there have been improvements to the uchime_denovo algorithm of vsearch. Specifically:

• uchime2_denovo
• uchime3_denovo

As outlined in the vsearch manual, these additional options are ideal for working with denoised amplicons. Also note that the minh option is ignored for these two options.

[mikerobeson]

Might be good to default to uchime3_denovo with option for abskew.

[colinbrislawn]

Supporting abskew is a good idea, but we may want to address that separately because

• abskew also applies to uchime_ref
• The only difference between uchime2_denovo and uchime3_denovo is their default abskew value,
  so I want to get --p-method working first