Support customizable user help format.

[remkop]

Ideally programmers should be able to specify a UsageFormatter interface implementation that gives complete freedom over the usage message layout.

Expose picoCLI usage layout functionality so custom formatters can reuse it.

Customizable:

• "Usage" line: mention option short form with [-v] optional and mandatory elements, or a generic [OPTIONS] [PARAMETERS]
• option columns: show in declared order or sort short options first
• option rows: sort alphabetically or show in declared order
• required options: show '*' in front of option name, or print 'Required' on 2nd line or print '(Required)' before or after the option description
• option parameter name: use field name or use field type (class SimpleName)
• option and parameter separated by space or by '='
• option parameter default value: print 'Default: VALUE' on 2nd line or print '(default: VALUE)' after the option description