Plugins for measured bandwidth and relay flags

This pull request adds two plugins:
1. Measured bandwidth AKA observed bandwidth AKA advertised bandwidth. Globe and Atlas show it as advertised bandwidth.
2. Relay flags.

I don't have a week's worth of graphs to add screenshots for the readme, but the pull request can hold for a few days until I do if that's desired.

Also added a missing shebang to circuits_by_country.py.