Allow reading options/globals from external config file (JSON)

JSHint allows feeding options/globals from a file via the `--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 string and passed like "normal" parameters (rather than `NativeObject` or whatever Rhino uses to represent an JS object internally).

I did my best to conform to the existing style and formatting, let me know if there's anything keeping you from merging this, I'd be happy to fix that.

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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.