gst-inspect: Colored output

gst-inspect: Colored output

Let's make the output a bit pretty to read. The colored output can be
disabled with `--no-colors` option or by setting `GST_INSPECT_NO_COLORS'
env (to any value).

The chosen colors are based on the popular Solarized theme, which is
targeted for both dark and light backgrounds.

Note:

* We only support true colors. If the terminal doesn't signal support for
  that via 'COLORTERM' env, we disable colored output.

* We don't add colors to --print-plugin-auto-install-info output, as
  that's meant for machines, not humans. Not only machines don't care
  about beauty, the existing ones will likely not expect colors and choke
  on it and we'll get angry mob at our doors.

[1] https://ethanschoonover.com/solarized