func WithSource(wrapped logrus.Formatter) logrus.Formatter {
	return &withSourceWrapper{
		wrapped: wrapped,
	}
}
WithSource allows to wrap a logrus.Formatter, to specify standard formatting options,
whilst adding the souce of the log, i.e. the caller.
package logging
import (
	"fmt"
	"runtime"
	"strings"
	"github.com/Sirupsen/logrus"
)
type withSourceWrapper struct {
	wrapped logrus.Formatter
}