when output filename is set, ANSI color codes are printed to stdout

[mjerabek]

Describe the bug
When Catch output is redirected to a file via the --out option and color output is forced via --use-colour=yes, the color escape sequences are written to stdout instead of to the file.

Expected behavior
The color codes are written to the file.

Reproduction steps

$ g++ -x c++ -DCATCH_CONFIG_MAIN -o tests - <<<'#include "catch.hpp"' # or really anything
$ ./tests -s -o results.txt --use-colour=yes >captured-stdout.txt
$ cat results.txt
===============================================================================
No tests ran

$ # no colors
$ od -t x1 captured-stdout.txt
0000000 1b 5b 31 3b 33 33 6d 1b 5b 30 6d 1b 5b 31 3b 33
0000020 33 6d 1b 5b 30 6d
0000026

Platform information:

• OS: Linux
• Compiler+version: GCC v8.2.1
• Catch version: v2.5.0

Additional context
I looked inside of the code and the reporters use Colour colour(Colour::ResultSuccess); as scoped colour setting. That calls Colour::use(Colour::Code) -> IColourImpl::use(Colour::Code). PosixColourImpl::setColour then retrieves the stream via Catch::cout(), which is wrong.

IMHO the cleanest solution is that the Colour RAII guard takes the stream, but that requires modifying all the reporters, even external (could be defaulted, but that would keep the code wrong unnoticed).
Alternatively retrieve the stream in setColour via getCurrentContext().getConfig()->stream(), which is much simpler, but I don't know if it could have any side effects.

[mjerabek]

The commit does not compile -- it should be getCurrentContext().getConfig()->stream(), but is getCurrentContext().getConfig().stream().