UserIO: StringPrompt improvements

- Edited StringPrompt to allow blanks if true is passed to it (false
passed as default value so as not to break current functionality)
- edited AddItem and EditItem to allow blanks
- Invalid quantity states will print as 1
- Removed '_' checking. Has been taken over by ListItem
- Minor comment changes
- endl adds for readability
Merge remote-tracking branch 'origin/master'

Conflicts:
	program_docs/ShoppingList/ShoppingList/Input.cpp