Change working mode

Instead of working on dicts, work on single secret values, and let the
user organize the data however they like.

New interface: ppillar -k <key> encrypt <mysecretvalue> > secrets.yml
               ppillar -k <key> encrypt @<mysecretfile> > secrets.yml
               ppillar -k <key> decrypt secrets.yml

More test files added.
Update README
Bump version to 0.2.0