feat(config): default config can be karma.conf.js or karma.conf.coffee

Before Karma would always set the config to `./karma.conf.js` (if not specified
otherwise). Now, it checks for `karma.conf.js` and `karma.conf.coffee`
and set the config file path ONLY if the file actually exists.
feat(runner): support config files

Closes #625
fix(runner): do not confuse client args with the config file

We have to split the args after `--`.
test(e2e): fix the pass-opts test