Add convenience method Help.fullSynopsis()

[remkop]

Add convenience method Help.fullSynopsis() as follows:

Help help = new Help(spec, cmd.getColorScheme());
writer.print(help.synopsisHeading() + help.synopsis(help.synopsisHeadingLength());

That would make this easier: #480

Also, add CommandLine.getHelp():

// CommandLine
/** Returns a new {@code Help} object created by the {@code IHelpFactory} with the {@code CommandSpec} and {@code ColorScheme} of this command.
 * @see Help#Help(CommandSpec, Help.ColorScheme)
 * @see #getHelpFactory()
 * @see #getCommandSpec()
 * @see #getColorScheme() 
 * @since 4.1
 */
public Help getHelp() {
    return getHelpFactory().create(getCommandSpec(), getColorScheme());
}