Closes #851

From npm audit fix --force

* fix(test): ensure that grunt test:client exits with non-zero code

Apparently it is not enough to check `code`, because when process
crashes `code` is set to `null` and `err` is passed instead.

Example of failing tests, which didn't fail build:
https://travis-ci.org/karma-runner/karma/jobs/537027667#L1046

* fix(test): adjust assertions to work in latest Chrome

Fixes #3339

* feat(runner): Return emitter that forwards server data

Allow the consumer to decide whether to echo the runner progress or not.

This might be the case when Karma.run() is called inside the same
process/terminal as the server is running in (e.g. development
servers).

Fixes #2121, #2799

* feat(cli): Echo the runner progress event, don't echo in runner

The CLI shall echo the progress, not the runner itself.

Fixes #2121, #2799

Between 3.x and 4.x we moved from combineLists to underscore.union in preprocessor.
Apparently combineLists was incorrect: the order of preprocessing changed in some cases.
Conceptually the order ought to depend upon the preprocessor, not the file.

Implement config.preprocessor_priority['preprocessor-name'] = priority, higher means run earlier.
Default priority is 0.

Add back compat API for karma-browserify.
Update docs.

The karma preprocessor caches file content: no need to read it async in the server loop.