Add an addRequiredOption method

[jrRibeiro]

This pull request adds an addRequiredOption method, which creates an Option with setRequired(true).

This is really useful, given the amount of projects I saw doing things like:

Options options = new Options();
options.addOption( "a", "all", false, "do not hide entries starting with ." );
options.addOption( "A", "almost-all", false, "do not list implied . and .." );
options.addOption( "b", "escape", false, "print octal escapes for nongraphic " + "characters" );
// ... more addOptions like these and then
Option stuff = new Option( "c", "stuff", true, "do not list implied . and .." );
stuff.setRequired(true);
options.addOption( stuff );
// And many more required options like this one

This pull request proposes an auxiliary method to create a required option, so that these options could just be added with.

options.addRequiredOption( "c", "stuff", true, "do not list implied . and .." );

[britter]

I have created https://issues.apache.org/jira/browse/CLI-267 for you.

[britter]

Fixed in svn and will sync to github shortly. Thank you!