Allow to specify a custom logger on Runner

    runner = Goliath::Runner.new(ARGV, nil)
    runner.api = Custom.new
    runner.app = Goliath::Rack::Builder.build(Custom, runner.api)
    runner.logger = CustomLogger.new
    runner.run

Signed-off-by: chatgris <jboyer@af83.com>
Add warn on logger options with a custom logger

When a custom logger is setup, option like `verbose`, `log_file` and
`log_stdout` will not have any effect.

Signed-off-by: chatgris <jboyer@af83.com>
Merge pull request #233 from chatgris/custom_logger

Allow to specify a custom logger on Runner