Not all reporters use a format that supports this, so TeamCity
and Automake reporters still do not report it. The console
reporter now reports it even on successful runs, where before
it only reported the rng seed in the header, which was showed
either for failed run, or for run with `-s`.

CLoses #2065

After everything else was split out, this name much reflects the
actual contents of the file(s).

This ended up being a surprisingly large refactoring, motivated
by removing a `const_cast` from `Config`'s handling of reporter
streams, forced by previous commit.

This way it makes much more sense from logically-const point
of view, and also means that concrete implementations don't
have to always have a `mutable` keyword on the stream member.

This reflects the renaming of the reporter/listener base class.

This describes the actual intention much better.

This means that the CLI interface now uses the new key-value oriented
reporter spec, the common reporter base creates the colour implementation
based on the reporter-specific configuration, and it also stores the
custom configuration options for each reporter instance.

Closes #339 as it allows per-reporter forcing of ansi colour codes.