Proposal for a contains rule

[funkyfuture]

while we were looking on a deceased project to test json-apis we noticed one validation rule it has that Cerberus doesn't have: contains. quiet obviously, it would test whether an object that implements a __contains__ method contains the constraining values.

# schema
foo:
  contains: "1"

# another schema
foo:
  contains: ["1", "2"]

# valid doc
foo:
  - "1"
  - "2"
  - "3"

# invalid doc
foo:
  - "4"
  - "5"
  - "6"

strings are not considered as a container object, so this isn't suitable to test substrings, there's regex for that.

or am i overlooking the possibilty to do such tests already?

[funkyfuture]

Implemented in #387.