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 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.

This includes always compiling the ANSI and None colour
implementations, as they don't need to touch any platform
specific APIs, and removing their respective compile-time
configuration options.

Because the Win32 colour implementation requires Win32-specific
APIs, it is still hidden behind a compile-time toggle,
`CATCH_CONFIG_COLOUR_WIN32` (renamed from `..._COLOUR_WINDOWS`).

The commandline options for colours were also changed. The
option now uses different name, and allows to select between
different implementations, rather than changing whether
the compiled-in colour implementation is used through
"yes/no/default" options.

Forcing it to be engaged explicitly, either via `op<<`, or by
`ColourGuard::engage`, fixes an issue with multiple `ColourGuard`s
being constructed in a single expression. Because the construction
of the `ColourGuard` instances can happen in arbitrary order,
colours would be applied in arbitrary order too. However, a chain
of `op<<`s has strict call orders, fixing this issue.

This also required splitting out Listener factory from
the reporter factory hierarchy. In return, the listener
factories only need to take in `IConfig const*`, which
opens up further refactorings down the road in the colour
selection and implementation.