Adds schema for better IDE completion, docs, and Intellisense

[cspotcode]

This PR adds a JSON schema for pug-lintrc files. The goal is to enable great code-completion and documentation within an editor or IDE. Here's an example of this in Visual Studio Code:

This is using a version of VSCode's puglint extension that's been modified to declare this JSON schema should be used for .pug-lint.json files. It's a 4-line declaration in package.json. After that, the schema and VSCode do all the work. Other editors probably support something similar.

Let me know if you have any questions!

[mrmlnc]

I think that the scheme should be and here: https://github.com/schemastore/schemastore/ 😃

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #121 into master will not change coverage

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[cspotcode]

The schema can be listed there, yes, but I think it should be generated here. Otherwise it won't be kept in sync with new rules and updated documentation.

schemastore.org can reference schemas that are hosted elsewhere. (for example)

[mrmlnc]

🏓 to @benedfit @TimothyGu 😄