imp: consistent da_barplot color scheme

[SergioAlias]

Closes #137 (I hope so!)

Now a color scale in da_barplot is specified, mapping "enriched" to #4c78a8 and "depleted" to #f58518 (these are the colors used in the default Vega Color Scheme for Nominal fields, #tableau10).

[gregcaporaso]

I confirmed that this works. Using this PR branch, here's the barplot generated on the Moving Pictures data:

When adding a filter, such that only depleted features remain, the color of those features stays consistent with the above plot:

qiime composition da-barplot \
  --i-data l6-ancombc-subject.qza \
  --p-significance-threshold 0.001 \
  --o-visualization l6-da-barplot-subject.qzv \
  --p-effect-size-threshold 4.3

Thanks for the PR @SergioAlias!