Support console.dir for pretty-printed object dumps when captureConsole=true

Sometimes when debugging I want to dump to console a pretty-printed object.

Normally in browser I'd use `console.dir`. In karma, this prints nothing, since `dir` is not listed [here](https://github.com/karma-runner/karma/blob/master/client/karma.js#L70).
I can use `console.dir` but the output is an ugly non-readable one-liner.

So the scope of this issue would be to
1) enable logging for `console.dir` when `captureConsole` is `true`
2) enhance a stringifier to be whitespace-aware (behing the flag), and pass that flag to the stringifier when logging from `console.dir`.
3) maybe also change the depth in case of `console.dir` to be more than `3`.

Would you be interested in receiving a PR with those changes?

Start in client/karma.js, where the issue notes that console methods are listed, then trace the console-capture path and locate the stringifier used for logged values. Done means console.dir is captured when captureConsole is true and can produce whitespace-aware pretty output, with any depth change validated against the existing console behavior and tests.

Written by the indexing model from the issue text.