Add support for IDefaultValueProvider

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]

I’ve given it some thought and I am leaning towards changing ArgSpec.defaultValueString() to return the value from the default provider (if one exists).

That meets the first goal (description text containing ${DEFAULT-VALUE} should be rendered with the default value), and at the same time allows application authors to distinguish between the default value that was programmatically set on an option (can be obtained with ArgSpec.defaultValue()) and the default value from the provider (can be obtained with IDefaultProvider.defaultValue(ArgSpec)).

(Also need to update the Javadoc for these ArgSpec methods to clarify where the values come from.)

[remkop]

I just realized we forgot about ${DEFAULT-VALUE} replacement in the usage help description. That is tricky because the ArgSpec does not have a reference to the default value provider… I guess we need to add that, or a reference to the CommandSpec, when an ArgSpec is added to the CommandSpec.