WIP: Adds support for string to collection coercion based on collectionFormat

[Deraen]

Fixes #69

TODO:

• We currently use collectionFormat multi as default in other parts of code: https://github.com/metosin/ring-swagger/blob/master/src/ring/swagger/json_schema.clj#L56, per Swagger/OpenAPI spec csv should be the default
• Coercion tests are broken, they use broken coercer function which returns matchers instead of coercer so they only run single layer of coercions instead of doing recursive coercion
• Would be nicer to set collection-format instead of collectionFormat. Maybe json-schema should do key transformations so that snake-case can be used here?

[ikitommi]

LGTM

[ikitommi]

This should be merged?

[Deraen]

Only if the todo items are implemented.

[ovan]

@Deraen Any plans to complete this? I'd be eager to use it. I guess the best alternative now is to spec the param to string and do parsing in the app?