[FEATURE] Add iframe coverage support

[matz3]

Enable iframe coverage reporting.
Add helper option to modify the karma config such that coverage is enabled in iframes.

the config gets enriched with

"coverageReporter": {
	"instrumenterOptions": {
		"istanbul": {
			"coverageGlobalScope":
				"(function() { var g=window;while(!g.__karma__&&g!==g.parent){g=g.parent;}; return g; })();"
		}
	}
}

when calling in file karma.conf.js

require("karma-ui5/helper").configureIframeCoverage(config);

after configuration has been set.

config.set({
	// ..
});

Can only be used in combination with the karma-coverage plugin.

Fixes: #138

[CLAassistant]

All committers have signed the CLA.

[matz3]

This fixes #138