tiltfile: add experimental_analytics_report

[landism]

Adds a new command that allows Tiltfile owners to report data to analytics to allow Tilt members to work with those Tiltfile owners to give them more insight into their team's Tilt use.

Sample Tiltfile usage:

username = local('whoami').rstrip('\n')
experimental_analytics_report({'user': username})

I'm not in love with the name, but it's less bad than a lot of the ones I came up with.