make it possible to run a group of tasks really easily

@laurieskelly suggested being able to use pseudotargets to run groups of tasks, kinda like tags

```yaml
---
# normalize and stem
---
creates: data/standardized_corpus.csv
depends:
  - src/normalize_and_stem.py
  - data/corpus.csv
command: python {{depends|join(' ')}} > {{creates}}

# tfidf
---
creates: data/tfidf.dat
depends:
  - src/calculate_tfidf.py
  - data/standardized_corpus.csv
command: python {{depends|join(' ')}} > {{creates}}

---
creates: something_awesome
depends:
  - data/standardized_corpus.csv
  - data/tfidf.dat
command: echo 'hi'
```

then you could just run `workflow something_awesome` to run all of the associated tasks