fedmsg-tail pretty args not pretty at all

[glensc]

  --pretty              Pretty print the JSON messages.
  --really-pretty       Extra-pretty print the JSON messages.

1. --pretty prints output in Python format, uncolored. prefixed with python log prefix
2. --really-pretty prints output in json, colored. when output directed to file, it's still with ansi escapes

this all is not intuitive, neither descriptive.

proposal:
deprecate them both, add --json, --python options (or --format=json, --format=python), for --json detect that output is not terminal and omit colors.

[jeremycline]

We can probably fix this as part of #424