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 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.
