Rule: valid-typeof

For a vast majority of use-cases, the only valid results of the `typeof` operator will be one of the following: `"undefined"`, `"object"`, `"boolean"`, `"number"`, `"string"`, and `"function"`. When a user compares the result of a `typeof` operation against a string that is not one of these strings, it is usually a typo. This rule ensures that when the result of a `typeof` operation is compared against a string, that string is in the aforementioned set.

    typeof foo === "strnig"

Fixes #715