Add experimental support for terminal colors (Linux-only).

When writing to std::cout or std::cerr, one can now use dedicated
stream manipulators to change the current font color.

Two API is provided:
- per-color API (red, green, ...)
- per-semantic API (fail, warn, ok)

When possible, please prefer semantics over color to ensure that the
user will have a consistent look'n feel.

This feature is currently only available on Linux.