Enable "routing" or "routines" to filter steps in a config file

[briandominick]

To go along with #14, once we have optional labels for the steps in a config file, we should be able to allow preset configurations of them. For instance:

routines:
  - name: no-pdf
    skip:
      - publish-book
      - publish-menu
  - name: preproc
    only:
      - migrate-assets
      - parse-menu
  - name: jekyll-only
      - config_preproc
      - jekyll-build

If this goes in the config file, we need to add a actions: block level or something to differentiate. Right now the whole file's top level is an array; that will need to be subordinated to actions: the way those above are subordinate to routines:. Can probably allow the current format, conditionally, as long as there are no routines declared.

Routines could be called from the command line:

bundle exec liquidoc -c _configs/build.yml -r jekyll-only

Also, routines may require a way to handle build-steps, which is probably most of what we'd want to filter. If they're named, we can list them as sub-steps of their parent using publish-book:pdf.

routines:
  - name: no-pdf
    skip:
      - publish-book:pdf