Add support for JSON schema

[yaodingyd]

Fix sindresorhus/electron-store#16

Use epoberezkin/ajv for JSON schema validation

[sindresorhus]

You need to consider how the defaults option will work with schema defaults. There's overlap. https://github.com/epoberezkin/ajv#assigning-defaults

[sindresorhus]

This PR is a bit thin considering the expected work for the bounty.

[sindresorhus]

The readme documentation should have a full realistic example of how it should be used. Meaning, various kinds of types and validations options.

[sindresorhus]

Also go through all the options in https://github.com/epoberezkin/ajv#validation-and-reporting-options and carefully consider which one to use or not.

[sindresorhus]

We should at least have these options: https://github.com/wulkano/kap/blob/01a557f6616108423f9e46718d48596a22698480/main/plugin.js#L48-L53

[sindresorhus]

Can you fix the merge conflict?

[yaodingyd]

I go through all the options for ajv:

1. We would only use basic and standard type and formats, no $data references, no custom formats;
2. We don't use referenced schema options(schemaId, missingRefs, extendRefs or loadSchema);
3. Except to useDefault, we don't want to modify validated data(removeAdditional or coerceTypes);
4. We don't use any advanced options except errorDataPath, which is good for reporting errors.
   So I think to use the same options makes sense.

[yaodingyd]

I was wrong about the overwrite order before: actually we apply Conf's defaults first, thus schema's default would not be used because value has already be set. I have updated doc accordingly.

[sindresorhus]

Can you add the option to https://github.com/sindresorhus/conf/blob/master/index.d.ts and https://github.com/sindresorhus/conf/blob/master/index.test-d.ts ?

And fix the merge conflict.

[sindresorhus]

Looks good now :)