Allow labels configuration in Rack middleware
Always set a label builder block
Merge pull request #4 from porras/feature/rack-collector-label-configuration

Allow labels configuration in Rack middleware
Clean up ruby code

Introduces rubocop, make it a mandatory part to pass the tests and fix
all offenses. The trailing comma detection is a bit weird, but having an
automatic linter/style checker should help to keep the code base clean.
Verify test coverage
Increase test coverage
Add status badges
Add support for text format
Test correct escaping