regular expression matching on files

from the design specification, the command line interface should have something like
```bash
workflow data/per_file --filename-root=a_specific_filename
```
to limit the `data/per_file` task to only depending on the file `data/first/a_specific_filename.dat` and only creating the file `data/per_file/a_specific_filename.dat`, as defined here:
```yaml
---
alias: data/per_file
creates: data/per_file/{{filename_root}}.dat
depends:
  - src/process_each_file.py
  - data/first/(?P<filename_root>\w+).dat
  - data/first/
command: python {{depends[0]}} {{depends[1]}} > {{creates}}
```