This looks a lot nicer.

Will make it easier to adjust this in the future, if desired.

The min size increase should at least prevent the labels from getting
squished together.

TODOs remaining:
-Preserve the bar width on changing taxonomic level
-When filtering the bar chart to show a certain taxon/level, ensure
 that this filtering is respected upon changing the bar width.

This is done by passing bar width to init(), which in turn is now
done by a "state" object that init() accepts as an argument.

It seems like some of the open problems in the bar plot visualization
(e.g. qiime2#91) could be addressed by storing more information about the
visualization's state. (Not sure just passing this around between
functions is the *best* way to structure things, though.)

Previously, changing the taxonomic level would revert the color
scheme back to the default (schemeAccent). Now, it should stick
with whatever the user had selected.

This is done by using the new "state" option of init(). I imagine we
could also use this in the future for outstanding issues like qiime2#87.