Add check for missing theme-check-disable comment

[jplhomer]

Related to https://github.com/Shopify/theme-check/pull/131#discussion_r563998142

When a user adds theme-check-enable but forgets to add a corresponding theme-check-disable (or, more likely, forgets to remove the former), this check would let them know so they can clean it up.

Might be some hairy ways around the implementation due to how the enabling mechanism works, but we should find a way to do this.