Allow reading options/globals from JSON file

JSHint allows feeding optins/globals from a file via `--config` parameter, it makes it easier to share the same configuration and reuse it in different tools (e.g. maven build and JSHint plugin for your favourite IDE)

Technical details:
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I tried to make as little impact on the existing code base as possible, that's why the JSON file is processed into a CSV key-value pairs and passed like "normal" parameters in a string form (instead of `NativeObject` or whatever Rhino uses to represent that internally).

I did my best to conform to the existing style, let me know if there's anything keeping you from merging this that can be fixed.

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Also, updated the README file, hopefully it's more readable now.


......I've just noticed that I accidentally included some "remove-trailing-whitespace" (my IDE's save action) inside `MojoTest.java`, I hope that's acceptable.