Add ImplicitDiv linter

Add the `ImplicitDiv` linter which checks for unnecessary uses of `%div`
when it would otherwise be implicit.

For example,

    %div.container -> .container

    %div#identifier.container -> #identifier.container

Change-Id: Id4f5bb361822531b8c6a3c06b3abcfbb5f6a32dc
Reviewed-on: https://gerrit.causes.com/28631
Tested-by: Shane da Silva <shane@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>