Plain Logging option

[winkler1]

Hi- I'd like a very simple logging format for when output is going to the Terminal (from unit tests). Would you be open to a PR with an option {plain: true} ?

Just simple stuff like -

var logAction = function logAction(ev) {
    if (!isLoggingEnabled(ev)) {
        return;
    }
    console.log(ev.type +" "+ ev.name + " " + JSON.stringify(ev.arguments));```

[kubk]

@winkler1 Sorry for bothering. If it's still relevant I've created a logger that allows to override the logger output: https://github.com/kubk/mobx-log