Add support for exclude directive in config files

Extend the configuration system to support an `exclude` directive, which
similar to the `--exclude` flag signals which files `scss-lint` should
not bother linting.

In the process support had to be added to properly update configs based
on the command-line flags that were passed to `scss-lint`, so that
combining the `--exclude` flag with a configuration file that defines
its own set of exclusions works as expected (i.e. the two sets are
merged).

Change-Id: I322f68d9319feb0abe17622c56133e0586f84c7c
Reviewed-on: https://gerrit.causes.com/31532
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>