barplot: add parameter for default sorting settings

[nbokulich]

Improvement Description
Add a parameter to barplot to define how barplots should be sorted by default (i.e., upon opening). Taxonomic level defaults could also be useful.

The idea here is that barplots can be created that can be posted in a paper or sent to a collaborator as-is, without needing to say "click on box X and sort by X and Y and Z".

Current Behavior
Barplots are level 1 and sorted by the most abundant taxon by default.

Proposed Behavior
Add a "default_sort" parameter, which would accept a list of terms.

Add a "default_level" parameter, to define which level to display on opening.

See q2-longitudinal's volatility for existing examples.

Comments
The only thing that would be waaaay more useful is if there is some way to save settings in a viz, but I recognize that's a pipe dream.