Support short form of clean, run, and scan value

For simple use cases, it should be possible to abbreviate the value for the clean, run, and scan operations. If the value is a string, it should be assumed to be the value of the main key. For example:

ext:
  collector:
    clean: build
    run: node generate-files.js
    scan: build

which is shorthand for:

ext:
  collector:
    clean:
      dir: build
    run:
      command: node generate-files.js
    scan:
      dir: build

The long form is only needed if other keys need to be configured.