require user to define the level delimiter

[gregcaporaso]

Fixes #121

This is a breaking change! Note for change log:

@gregcaporaso addressed an issue in q2-composition da-barplot where the visualization made 
assumptions about the feature id schema. The split feature ids on semi-colons for readability in 
figures, users should now provide the `--p-level-delimiter ';'` parameter (where previously this 
was not required).

[Oddant1]

lgtm