Add Favourites to StudySpot

Concept: a FavouriteCommand that allows us to save certain locations into a list of favourites.

Possible syntax: fav n/Bishan Library

Result: Added 'Bishan Library' to favourites!

We should be able to remove locations from favourites as well.

Listing all locations should show showing if a particular location is in Favourites.

Example (using the heart Unicode character, ♥, to show that Bishan Library is in favourites):

list

1. Bishan Library (♥) ★★★★★ [aircon][wifi] 
#not-crowded

2. SRC Utown ★★★★★ [aircon][wifi]
#crowded

Implementation:

Should we limit the number of favourites that a user can have? E.g. a User can at most have 5 favourited locations.
Should we have a fav and unfav command? Or should fav toggle the favourite status of a location?

Potential future enhancements:

Filter by favourites (#45)
To show the list of favourites in the GUI, similar to the ObservableList and how it changes when filters are applied.

AY2122S1-CS2103T-T09-1 / tp