Add no-universal-selectors rule

[bgriffith]

What do the changes you have made achieve?

Adds a new rule no-universal-selectors

Have you written tests?

Yep

Have you included relevant documentation

Yep

Which issues does this resolve?

Relates to #694

<DCO 1.1 Signed-off-by: Ben Griffith gt11687@gmail.com>

[coveralls]

Coverage increased (+0.01%) to 97.109% when pulling d4e5270 on bgriffith:feature/universal into 098de67 on sasstools:develop.

[DanPurdy]

This doesn't read right, Will enforce the use of universal selectors or enforce whether universal selectors can be used.. Prevent the use of... Something more like that?

[coveralls]

Coverage increased (+0.08%) to 97.181% when pulling 1ebfffe on bgriffith:feature/universal into 098de67 on sasstools:develop.

[coveralls]

Coverage increased (+0.01%) to 97.181% when pulling 7b29503 on bgriffith:feature/universal into 33e8d84 on sasstools:develop.

[DanPurdy]

👍

[coveralls]

Coverage increased (+0.01%) to 97.191% when pulling ed42f11 on bgriffith:feature/universal into 0c19488 on sasstools:develop.