When the added Bazel configuration flag is enabled,
a default JUnit reporter will be added if the XML
envrioment variable is defined.
Fix include paths for generated config header.
Enable Bazel config by default when building with
Bazel.


Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

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 is useful as a centralized place for handling common reporter
problems like handling output streams, and soon also colour impl
handling.