add support for generating bar plots from ANCOM-BC data

[gregcaporaso]

This pull requests adds first pass visualizers for addressing #104, inspired by Figure 6 of the ANCOM-BC paper.

An example .qzv can be viewed here. The example data is from the Human Microbiome Project (HMP 1). It was accessed using Qiita under study id 1928 in 2022. Differential abundance was computed using ANCOM-BC via q2-composition with QIIME 2 2022.11.

Still to-do:

• [x] add unit tests
• [ ] ~add usage examples~ (likely won't be able to get to this in time)
• [x] add new altair dependency to package

[gregcaporaso]

This one should be at a state that is good enough for an MVP of this type of visualization. The main index page and HTML generation generally could use some work (e.g., to have tabs for the different plots rather than links, and to template the whole page) but the code is pretty well unit and field tested.

[gregcaporaso]

The link to the example viz above has been updated after commit c230df8, so gives a current view of what this looks like (that link again is here).

[ebolyen]

Just waiting on tests for merge.