Add peer deps and missing ajv formats

[amh4r]

Great library! Here are fixes for some snags I hit.

• Add ajv and ajv-format as peerDepedencies. They're needed to consume the generated files, so I thought that peerDependencies was another way to communicate that. The specific version of ajv matters, and peerDependencies seems like a good way to point that out.
• There was a breaking change in ajv@8.0.0 which renamed ErrorObject.dataPath to ErrorObject.instancePath. The changelog can be found here.
• I got unknown format warnings for date-time, int32, and int64. The date-time warning was solved by ajv-format, but the int32 and int64 warnings weren't. However, int32 and int64 are sitting in master (see this PR), so they should be in their next release.

[timvansteenis]

@goodoldneon Sorry for my late response and thanks for this PR!

I just published version 2.2.0 which contains support for ajv-formats. I've used this PR for inspiration so thank you for that!

I've added:

• ajv-formats support
• peerDependencies

ErrorObject.dataPath to ErrorObject.instancePath was renamed in version >= 2.0.0

Note: the output structure and generation options have been changed from version >= 2.0.0. Make sure to remove your output folder before generating.

I will be closing this PR.