Add TagCommand functionality

[Kalsyc]

Modifications: that will resolve #23.

1. Add TagCommand functionality.
2. Edit UserGuide to include TagCommand.
3. Xpire now reads trimmed version of the firstWord when parsing.
   (i.e. add |banana|10/10/2019|#Fruit is valid)

To do:

1. Add J-Unit Testing for TagCommand and TagCommandParser.
2. Fix J-Unit Testing such that tag|1|Fruit#Food should only read #Food as the sole tag of the item at index 1 and not both #Fruit and #Food
3. Add more documentation and methods (e.g. equals method) for defensive measures.