Check for null date before attempting to print

#64
Merge commit '8ec60a9a1deeab97c03e4b7b1e33f4bb0b29fbaa' into ui/victor
#64
Added more validation for input

#63
#51
Merge branch 'ui/victor' into parser/Victor
Refactored code for printing priority
Removed DONE Additional Argument Type
Added validation for Deadline and Priority additional arguments

#51
Refactored Code
Added new keywords
"dd" and "dl" for deadline
"ds" for description
Supports --done to be parsed as --tag done
Priority medium -> normal and make it default
Added toString() for Additional Argument
Added toString for Command
Added test cases for parseToDate
Merge branch 'ui/victor' into parser/Victor
Merge commit 'fbf147aa057051b1fd49da08ad1359e3986fc9fc' into parser/Victor
fix bug in handler
Catches the event whereby a single input tries to set both the deadline and time range. #65
Included validation for input dates and times #51 #29
Included validation for parsing to priority #51
Included validation for add command to disallow name additional argument #51
Refactoring unit tests #49