Added color to output. Changed the behaviour for empty task lists.

Changes:

1. Several color codes are defined in the class textcolor.
2. Empty task list behaviour is changed: (tasky l -a AND tasky l -s)
In the previous version, when the list is empty, the code simply write the lname of the empty list + '(empty)' and exit. With the current version, it will continue listing the other task lists.