[Feature Request] Allow to to emit colored click.secho() text to a pipeline.

Currently click.secho() strips the color information when the output stream is a pipe and not a terminal. With this feature, it will be possible to force click.secho() to preserve the colors even if writing to a pipe. This can be done at several levels and each one of them should work. For example having the functions below, at a global or per stream level. This will allow the parent process to pass the color policy to the subprocess via command line args.

enum ColorPolicy = ALLWAYS_KEEP | ALLWAYS_STRIP | AUTO

# Used at child process.
click.set_color_policy(color_policy:  ColorPolicy)

# Used at parent process.
click.get_color_policy() -> ColorPolicy                    

We have a python program that runs a scons script as a subprocess, captures it's stdout pipe, and sends it to a terminal. When we use ``click.secho() in the subprocess, the colors are removed. This result with the non desire outcome that some our output is colored and similar output that came from the subprocess is not.

I am not aware of a workaround.

Start by tracing how click.secho() decides whether to emit color when stdout is a pipe rather than a terminal. Compare the requested global and per-stream color policies with the subprocess use case, and clarify how AUTO, ALWAYS_KEEP, and ALWAYS_STRIP should behave. Done means a documented, tested policy API can preserve or strip colors for piped output.