spec_runner.html: allow to specify require_config.js (by URL parameter)

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For testing widgets independent of their install-location, we'd like to help the user to specify the location of the require_config.js without modifying the spec_runner.html.

This could be implemented through a URL parameter, but we need to make sure that this parameter cannot be abused to execute arbitrary scripts under the origin of the spec-runner's site.

For example, we could allow only a number of levels to go up (-1 might go straight to the root).

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To stay safe, we can allow to specify a numeric depth level.
The level is used to look up the require_config.js relative to the location of the spec_runner.html. The default would be 4 (../../../../require_config.js).

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depth -> folderDepth

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Implemented on master (v0.4.0).

Upgrade Information

The example HTML spec-runner (for browser-use only) now allows to specify where to look for the require_config.js.

By default, the configuration will be searched as if the spec-runner was used by a widget installed under includes/widgets/{category}/{widget-name}. To use a different location, pass the URL parameter folderDepth={x} to the spec-runner, where x is the number of URL segment to go upwards for finding the require_config.js.