Add Option.Parse<T>() method; cleanup.
Add Option.Parse<T>() method; cleanup.

	* src/NDesk.Options/NDesk.Options/Options.cs:
	  - Move `private T OptionSet.ConvertFromString<T>(string, string)` to
	    `protected T Option.Parse<T>(string, OptionContext)`.  This makes it
	    easier for subclasses to do generic type parsing w/o code duplication.
	  - Code cleanup; remove unused variables, constructors, etc.

MikeJansen/NDesk.Options.StrongNamed,chakrit/ndesk-options-mirror,MikeJansen/NDesk.Options.StrongNamed,chakrit/ndesk-options-mirror,MikeJansen/NDesk.Options.StrongNamed