text plugin: add color support

This patch adds terminal color support to the text plugin.

The text component class accepts a new initialization parameter `color`
which you can set to one of the following strings:

never:
  Never output color codes.

auto:
  Output color codes if the standard output is connected to a
  color-capable terminal and if the file stream to use is the standard
  output.

always:
  Always output color codes. This is useful to print colors even if the
  standard output is not connected to a terminal, for example:

      babeltrace /path/to/trace -o text.text -p color=always | less -R

The semantic of this parameter is the same as ls(1)'s and grep(1)'s
--color option.

The chosen color scheme is open to debate. In an ideal world, it should
be configurable by the component parameters, or read $LS_COLORS.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>