Proposal: command line support to run focused tests

Hello, this question comes across every time when I work with Karma+Jasmine in my Angular projects. Yes, we have `fit` and `fdescribe` but I personally find this error-prone solution, where you have to modify test files and revert it back after changes.

Last time I came to a solution where I modify karma.config for the interested set of files. Meanwhile, I have some experience with Jest and I found its [files pattern](https://facebook.github.io/jest/docs/en/cli.html) very lightweight and elegant. You just type `jest yourcomponent` and it runs matching test files.

I want to propose to add this feature to Karma as well, because it's really needed and missed.

Start by reading Karma's command-line handling and the karma.config configuration path, then compare the proposed file-pattern behavior with the linked Jest CLI documentation. Done means users can provide a pattern from the command line and Karma runs only matching test files, with coverage for the new behavior.

Written by the indexing model from the issue text.