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 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.

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.

This opens path to per-reporter colour output customization,
and fixes multiple issues with the old colour implementation.

Under the old implementation, using Win32-backed colouring
would always change the colour used by the console, even if the
actual output was written elsewhere, such as a file passed by
the `--out` flag. This will no longer happen, as the reporter's
colour impl will check that the reporter's stream is pointed
to console before trying to change the colours.

POSIX/ANSI colour implementation suffered a similar-ish issue,
in that it only wrote the colour escape codes into the default
output stream, even if the reporter asking for colouring was
actually writing to a completely different output stream.

This will become useful when reworking colour support, because
Win32 colour support requires checking whether the output is
stdout, which is done through the `IStream` wrapper.