Make babeltrace(1)'s CLI Git-like and implement the list-plugins command

Make babeltrace(1)'s CLI Git-like and implement the list-plugins command

This patch adds a command concept to the babeltrace(1) program, so that
the usage becomes:

    babeltrace [GENERAL OPTIONS] [COMMAND] [COMMAND OPTIONS]

The idea is to support multiple actions performed by the same program
without having to deal with a single command-line option parsing phase.

The default command is `convert`: it builds a trace conversion graph and
runs it. Therefore the current behaviour without a command name remains
the same.

The general command-line options, which can apply to all commands, are:

    -d, --debug        Enable debug mode
    -h  --help         Show general help and quit
        --help-legacy  Show Babeltrace 1.x legacy help and quit
    -v, --verbose      Enable verbose output
    -V, --version      Show version and quit

You can get the usage of a specific command with:

    babeltrace COMMAND --help

Because there is a default command, the parsing of the general options
is done manually to catch the first unknown option OR positional
argument. If an unknown option is found, all the arguments are given
back to the convert command. This is why the convert command also
parses --verbose, for example, because of this situation:

    babeltrace --debug --sink=my.sink --verbose

Here, the general options parsing stops at --sink because it's unknown,
and no explicit command name was found so far, so everything starting
with --debug is given to the convert command.

I also implemented the `list-plugins` command which lists plugins, their
component classes, and their properties:

    babeltrace list-plugins

You can still specify a plugin path or use the BABELTRACE_PLUGIN_PATH
environment variable with this command:

    babeltrace list-plugins --plugin-path=/path/to/other/plugins

I added color support in libbabeltrace-common and used it in the
`list-plugins` command. Color codes are only printed when color support
is detected, that is, when the standard output is connected to a
color-compatible terminal.

I also improved the way bt_value objects are printed with print_value()
in babeltrace.c to make it look more YAML-ish. This is easier on the
eyes.

This patch introduces a backward compatibility break in the very
specific scenario where an explicit command name is used and a directory
in the CWD exists with this name:

    babeltrace list-plugins

If `list-plugins` is a directory in the CWD, the legacy behaviour is to
recursively find CTF traces in it. With this patch, plugins are listed.
However a message is printed at the end of the command in this case:

    NOTE: The `list-plugins` command was executed. If you meant to
    convert a trace located in the local `list-plugins` directory,
    please use:

        babeltrace convert list-plugins [OPTIONS]

In other words, `babeltrace convert` (with 2.x) is a drop-in replacement
of `babeltrace` (with 1.x).

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