IMP: expose 'strand' argument to exclude-seqs

[gwarmstrong]

Improvement Description
For the plugin, incorporate a strand argument to be fed to blast or vsearch that can limit whether the reverse complement is aligned against: 'strand': Str % Choices(['both', 'plus'])
This is invoked in the following lines

https://github.com/qiime2/q2-feature-classifier/blob/35a518104a2f71fe2c72e0897345ccfe776dfadc/q2_feature_classifier/_vsearch.py#L131-L137

And I think makes reasonable sense to include as a functionality in this plugin.

Current Behavior
Only both strand search is enabled for vsearch and blast

Proposed Behavior
Provide an additional parameter that allows searching the plus strand with these commands.

Questions

1. Do you think this feature is a reasonable fit for this plugin?