Add no-attribute-selectors rule

[bgriffith]

What do the changes you have made achieve?

Adds the no-attribute-selectors rule

Have you written tests?

Yep

Have you included relevant documentation

Yep

Which issues does this resolve?

Related to #694

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

[coveralls]

Coverage increased (+0.009%) to 97.106% when pulling a32fe32 on bgriffith:feature/no-attribute into 098de67 on sasstools:develop.

[coveralls]

Coverage increased (+0.08%) to 97.178% when pulling ce51219 on bgriffith:feature/no-attribute into 098de67 on sasstools:develop.

[coveralls]

Coverage increased (+0.009%) to 97.2% when pulling dd1d0cf on bgriffith:feature/no-attribute into 65e4354 on sasstools:develop.