use YAML not JSON for .counter.dat

[icornelius]

The --counter option assigns a serial number to new notes created with the zettel command. Usage:

--counter COUNTER     counter name (defaults to --id if present)

The file .counter.dat maintains the count of zetteln written for each COUNTER. This is serialized in JSON.

I suggest switching to YAML since that will introduce newlines in the file and simplify resolution of git merge conflicts in distributed projects. Alternatively, users could add .counter.dat to their .gitignore. Then each local system would keep its own separate count, defeating the purpose of a counter.

[icornelius]

The relevant commit, setting counter output, is d39210b. See also 02de204.

A final newline should be written to the end of .counter.dat, to facilitate display in the terminal (e.g., with cat).

[icornelius]

1989571 is sufficient because yaml.load reads any existing json-encoded .counter.dat on first invocation. There is no need for a dedicated one-time translation between formats, as yaml is a superset of json.