Add support for pattern properties

[theory]

Convert the patterns to actual RegExp objects and then render them as such. This is especially useful for the perfectly valid empty pattern, which otherwise would not appear.

Add a function, propertySpecs, which returns specs for handling both properties and patternProperties, and then iterate over them both to produce both the summary table and the more detailed info. Right now it creates a separate table for patter properties, but it might be worth considering a single table, since the rendering of the regex makes it pretty clear which is which.

Add tests for pattern properties, too.

I suggest hiding whitespace changes in the diff view, as most of the changes to lib/generateMarkdown.js are indentation.