feat: add istanbul ignore flag to configuration object

[westpole]

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[westpole]

I signed it!

[dignifiedquire]

If we make it required, depending on ignore we should either have a default or throw an error when it doesn't exist so users know what the problem is.

[westpole]

Actually, the idea was not to make it required. If "config.istanbul.ignore" equals to undefined or false, then output will get an empty string. So, I do not need to specify any parameter in karma configuration if I do not use istanbul coverage.

[dignifiedquire]

Yeah, totally right, sorry -.-