chore(KarmaConf): read CLI browser option to run tests

[ghybs]

Hi,

This PR (re)makes it possible to specify a browser when executing a test command (e.g. npm run test-nolint -- --ff for Firefox).

So that it is easier to run tests in local browser (other than PhantomJS).

This CLI argument browser option disappeared when Jake has been removed from scripts.

This PR will make it easier to locally debug to re-enable PR's #5845 and #5831

[cherniavskii]

Hmm, IIRC it's possible to do this right now, e.g.:
npm test -- --browsers Firefox,Chrome,Safari,IE

[ghybs]

mmm I got caught by the same gotcha as in my PR:
https://github.com/ghybs/Leaflet/blob/ecd1c009ce4420f1d96b285343bc42560431eb19/spec/karma.conf.js#L134-L136

=> use either npm test -- -- --browsers Firefox (twice -- flag before --browsers)
or npm run test-nolint -- --browsers Firefox

…because of the nested npm script, we have to use multiple times the end of args flag "--"

Thanks for the quick review!