Add support for loading specific config file

Add a `--config` flag which is used to load a configuration file
specified by the user.

In the process, a couple of specs had to be shuffled around as we are
now checking the `Config` object rather than a plain options hash in
order to find out if a particular linter is enabled.

For the moment, this enables basic support for disabling linters via
a configuration file, rather than having to specify them via the command
line.

Note that the command line options will always trump any config options
set via a config file, even one specified with the `--config` flag.

Change-Id: I944f6e71dbf892491be70b8f85dc3b61bb0eb9ed
Reviewed-on: https://gerrit.causes.com/30920
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>