Update syncrules
Update syncrules

- add diffing
- update based on file diff, not date, assuming snakemake_rules is the
  correct version
- enable syncing of one rule/file only

Fixes #29

percyfal/snakemakelib-rules,percyfal/snakemakelib-rules,percyfal/snakemake-rules,percyfal/snakemake-rules,percyfal/snakemakelib-rules