Add support for vocabulary display

Parallel to the current nested taxonomy display, it would be desirable to display vocabulary information. This could follow the new taxonomy view url format: taxonomy/term/4. So, e.g., taxonomy/vocabulary/1 would present a page with page title of vocabulary 1 and description of vocabulary 1 followed by first-level terms of vocabulary 1. Something similar is implemented in the summary module using taxonomy__context to display first-level terms of a vocabulary. Ideally, perhaps, this would also honour a depth parameter as per the term display, so that taxonomy/vocabulary/1 would display only the vocabulary name and description, taxonomy/vocabulary/1/1 would display that plus first-level child terms.