Configure progress reporter to report after X number of executed specs

It would be nice to be able to configure the progress reporter to only report after X number of executed specs instead of after every one.

This would be especially useful for projects that have large number of unit tests that run in an environment that doesn't support escaping sequences. For example, I work on a code base that currently has over 8,000 unit tests which writes out over 100 lines of dots.

I'd imagine a new entry in karma config

```
// configures progress reporter to only report
// after the execution of 500 specs
reportThreshold: 500
```

Start by locating the progress reporter and the Karma configuration handling. Trace how executed specs currently trigger progress output, then verify that a configurable threshold controls reporting for large test suites without escaping-sequence support.

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