Automatically inject taxonomies into generators

[chriskrycho]

The generator CLI should support integrating data from the specified site configuration to specify flags for taxonomies. Moreover, it should infer default long-flags for all taxonomies in a specified site. I.e. if I have defined the taxonomies Category, Tag, and Section, I should be able to type lx new post --subject Waffles --tag food --Category Life without having to do any further configuration. Moreover, I should be able to specify the short flag in the configuration, so that those could be shortened to -c, -t, and -s (or whatever else) respectively.