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Show synopsys, help invocations and the like

Require the nice docstring_parser package which supports a bunch of
formats.

- recursive Group.load()
- preventing from importing stuff from above level
- preventing recursive same-level imports
- rely purely on the help command in cli2.cli
- un-needed trailing whitespaces removed from output

Multiple aliases allow to specify a list of aliases in the alias
argument.

Same for the negate argument.

Also, added a POSIX feature that may be enabled at the Command or Group
level.

If a non-bool alias starts with a dash, then a space may replace the
equal: `-a=b` is same as `-a b`

BC BREAK: Group.cmd was renamed to Group.add because Group.cmd is now a
decorator

Decorators added:

- cli2.cmd
- cli2.arg
- Group.cmd
- Group.arg
- Command.cmd

Possibility to change the class of a Command or Argument was also added.

The implementation of console_script really is **not** meant to be
compatible with usual python callables, the script it generates
**requires** the callable function to return an exit code.

This is what a generated console_script looks like:

    sys.exit(
        load_entry_point('cli2', 'console_scripts', 'cli2')()
    )

As such, this commit introduces the following changes:

- Both now inherit from the new EntryPoint class which just defines an
  entry_point method that you can use in console_scripts (see README)

- You must change your console_scripts from foo:bar to
  foo:bar.entry_point

- As such, EntryPoint.entry_point is in charge of
  __call__(*sys.argv[1:])

- Command and Group __call__ now always take *argv, still return the
  target callable result since that's now possible, which makes the test
  code even nicer !

- It was decided that if you want to exit with non-0, then either raise
  an Exception (duh!) either call sys.exit(1) yourself.

- colors
- overrides
- sub-group auto-load
- dochelp niceness

Test & Document driven
Fruit of 3 rewrites from scratch
Keeping the best of each ...