Update and rename add_groups.cpp to main.cpp

-The file was renamed because I will be adding helper .cpp files into this program and adding onto a program window prompted by the cygwin screen. Also, main.exe is easier to type than add_groups.exe.
-I got ATOI to work! :D No more long helper function
-When I update the number of watchers each group has every month, there is a specific function that lets me go line by line and plug in the number of watchers rather than go through a .txt file and figure out categories for myself.
-Also accounts for adding a new type of input in which numbers prefaced by a semicolon (i.e. ;930) are read as the number of watchers a group has and are categorized accordingly.