add check which complains about non-compliant TODOs

[robstoll]

e.g. if issue indicator is proj#\d+
then it should complain if it finds one of the following TODOs:

• TODO blabla
• TODO #123 blabla
• TODO blabla proj#123

I suggest we add tje flag --require-issue-number which enables this check. I.e. one should be able to use e.g. todo-checker -i "proj#123" which checks only for a particular issue and this check would not kick in and complain about other todos.
This way one could use:

todo-checker -i "proj#123"; todo-checker -i "proj#\d+" --require-issue-number

to check both