Add custom conditions use cases to the docs

[myovchev]

Details

• Add custom conditions use cases to the docs
• Add tests for corresponding examples
• Add version requirements for every use case in the documentation
• Propose unified terminology to avoid confusion:
  core condition: AND, OR. NOT, etc
  custom condition: declared and registered custom condition conditionName, available in the grand flow as custom:conditionName
function condition: native JS function, added while defining the grands, not serializeable
• Expose getValueByPath from condition utils to allow developers to use JSONPath in their custom condition implementations. It's not exposed in the browser build (./app.js), but I'm not sure how it'll affect the bundle size.

Ref: #45

[codeclimate]

Code Climate has analyzed commit 68c4445 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 90.6% (0.0% change).

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[koladilip]

@myovchev Thanks a lot for this change.