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

tools/gst-inspect-1.0.1

@@ -57,6 +57,10 @@ is at least the version specified
 .B  \-u, \-\-uri\-handlers
 Print supported URI schemes, with the elements that implement them
 .TP 8
+.B  \-u, \-\-no\-colors
+Disable colors in output. You can also achieve the same by setting
+\'GST_INSPECT_NO_COLORS\' environment variable to any value.
+.TP 8
 .B  \-\-print\-plugin\-auto\-install\-info
 Print a machine-parsable list of features the specified plugin provides.
 Useful in connection with external automatic plugin installation mechanisms.