fix(config): Default remaining client options if any are set

[firesock]

Closes #961

[nemoDreamer]

🐼

[firesock]

@maksimr Mainly wanted to keep the defaults in the same place in the file and access in a similar way to other defaults. Is it preferable to just use a simple top-level variable?

[dignifiedquire]

lgtm

[maksimr]

@firesock all properties of instance Config user can change in karma.config.js, I dont think make defaultConfig as public property of config good idea.

Thanks!

[dignifiedquire]

#merge

[karmarunnerbot]

Sent to presubmit as presubmit-master-pr-1357

[firesock]

Sorry, was away for a while, couldn't pick up the comment. It was merged anyway, so I assume that's fine. Let me know if you still want the changes?