Add `subtype` and `editable` fields:
- `subtype`: to distinguish between regular strings, file paths, dir paths,
etc.
- `editable`: To indicate a setting should be editale after submitting the
job. Editing will trigger re-compilation of the job.
Each job compiler script now must define a `compileJob(job)` function,
which will be called by Flamenco when necessary.
This makes it possible to run the script without a job, and get other
exported symbols from it, such as metadata about which settings its job
type needs/exposes.
