Fix the help option

Prevent multiple usage of the 'help' option.
Fix performance ReplaceLetter method in HelpCommand class

Replace N complexity with constant complexity.
Revert
Merge branch 'master' of https://github.com/klimentt/Hangman-6
Merge branch 'Branch_08911354295c5a9ff7c9e4bdd4c641122330e35f'

Conflicts:
	Hangman-6.v11.suo