Added option to avoid that `gulp` process ends on a compilation error

I added a new option `logErrors` to change the behaviour when a compilation error happens. Right now if a compilation error happens then the plugin report the error to `gulp` calling the callback with it and `gulp` abort the process due this.

There are situations where you don't want that behaviour, you want to keep the process running and you only want to know that an error happened, for example if you are watching changes in jade files because you are developing, you don't want that a compilation error stop `gulp` watch process, you want to see the error to fix it and recompile again so keep `watch` process running, as I've also commented in the README.