Make the temp folder configurable

Karma currently (1.7.1) uses the default system temp folder, as evidenced here:
https://github.com/karma-runner/karma/blob/39d378d20dee61a88572225db9e1855da3adb346/lib/temp_dir.js#L7

It would be useful for this to be configurable (e.g. on our build environment the /tmp folder has a limited size, so we are running into some problems).

Start with lib/temp_dir.js at the referenced line to understand how Karma selects its temporary directory, then trace where that directory is used. The work is done when users can configure the temporary folder and the existing default behavior remains intact; add or update tests for the configuration if the repository has coverage for this area.

Written by the indexing model from the issue text.