Add support for IDefaultValueProvider

[remkop]

Add support for IDefaultValueProvider on @Command and potentially on individual @Option and @Parameters.

See discussion under #261.

Proposed API:

interface IDefaultValueProvider {
    /** Returns the default value for an option or positional parameter or {@code null}.
     * The returned value is converted to the type of the option/positional parameter
     * via the same type converter used when populating this option/positional
     * parameter from a command line argument.
     * @param argSpec the option or positional parameter, never {@code null}
     * @return the default value for the option or positional parameter, or {@code null} if
     *       this provider has no default value for the specified option or positional parameter
     */
    String defaultValue(ArgSpec argSpec);
}

[remkop]

Note to self: this ticket can/should be implemented before #322. Given the discussion on #280, there are some trade-offs with a defaulValue annotation attribute, while the benefits of this ticket are more clear.