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.

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 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 makes sure that CAPTURE works when called with variadic arguments,
and also works that way when disabled.

The underlying fix to #2316 is not applicable (CAPTURE is already
variadic when disabled).

This is a port of 5e94498ed04820b1022f0bead0f7d4d5fb7a06dd to the
devel branch.