Enforcing a single whitespace between colon and commit subject

Hey there!

Commitlint's recommended rules include [subject-case](https://github.com/conventional-changelog/commitlint/blob/master/docs/reference-rules.md#subject-case) checking that the subject is not sentence-case, start-case, pascal-case, upper-case. However this rule is easy to bypass by adding a leading whitespace in the subject. From the user perspective it's almost impossible to see if there are one or two whitespaces:

`feat: MY SUBJECT` // two whitespaces between `feat:` and `MY`, bypassing subject-case

and it discards the benefits of the `subject-case` rule.

Please consider adding an extra rule that enforces that subject must not start with a whitespace.