Lint names of mixin and function declarations

Previously, only variable declaration names were linted to ensure they
were lowercase and hyphenated. Change this to also lint mixin and
function declarations.

This removes the old `VariableNameLinter` and replaces it with the
improved `DeclaredNameLinter`.

Closes #8.

Change-Id: Icf250cdf1ac79ea4022a0c13635444d9cf020095
Reviewed-on: https://gerrit.causes.com/23138
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>