[REQUEST] Avoid failed network request to fetch runtime config file

[paulwarren-wk]

What should we change and why?

The viewer currently attempts to fetch a file called "ixbrlviewer.config.json" from the same directory as the script. The config file is not required, and if the attempt to fetch it fails, this results in an error logged on the console but the viewer loads normally.

These failed network requests may be undesirable, as they suggest that something is broken when it isn't.

Possible solutions:

1. Provide a mechanism to disable loading of the runtime config. This could be a viewer "feature", although that's potentially a bit confusing, as we allow features to be defined in the runtime config.
2. Recommend providing a config file in the documentation. A file containing an empty JSON object would be sufficient.
3. Do a HEAD request to see if the file exists before attempting to download it. Not sure that this really fixes anything, but perhaps makes it clearer that we just checking to see if the file exists it's OK if it doesn't?