v0.7.0: one question on first run, so we can see what Squidbrake catches

The first time you run squidbrake in a terminal, it asks one question (Enter means yes):

Squidbrake can send anonymous usage stats to its team:
  - which squidbrake commands run, the version, OS and country
  - counts of what it checked, held and blocked: the program only (e.g. "rm"), the rule, and sizes
Never your commands, files, prompts, rules or keys. It shows one small team what to fix next.
Send anonymous usage stats? [Y/n]

• Asked once. Never asked in the agent hooks, scripts, CI or with --yes, and nothing is sent from them until someone has said yes.
• squidbrake telemetry status shows exactly what is sent; squidbrake telemetry off (or SQUIDBRAKE_TELEMETRY=0, or DO_NOT_TRACK=1) stops it.
• squidbrake register you@company.com tells the team who you are, if you'd like help rolling it out (asks first).
• An install already in a pilot keeps its pilot.

Details: telemetry.py and pilot.py.