BREAKING CHANGE: this changes configuration schema for custom label values, it provides more flexibility when setting custom labels and also prevents '*' special wildcard value from conflicting with labels, since '*' is a valid label value.
Fixes#537
Right now backend sends both background and font color information, refactor to send background color and background brightness level (needed to pick font color) and let UI apply font color. Needed to implement #18
This adds support for reading configuration from file, env support is still there and legacy env variables will still work, but flags are now following config schema, so they don't match old flags.
Having a config file allows to express more complex configuration options, which is needed for some feature requests.
Internal packages are supported by Go 1.5+, any package in /internal/ dir is only importable from the same repo. This will cleanup main dir a bit and provide better namespace for unsee subpackages
