title: Use circe to preprocess dot-notation style fields
body: <p>I have some json that includes some fields that are being flattened into a bson-ish format as in <code>{"foo.bar" : "bash"}</code>. I'd like to transform this to the following representation <code>{"foo" : { "bar" : "bash"}}</code> and wondering where in circe I'd do such an operation.  Complicating the problem is that there could be multiple such fields that need to be properly merged, e.g. <code>{"foo.bar" : "a", "foo.bash" : "b", "foo.baz" : "c"}</code> -> <code>{"foo" : { "bar" : "a", "bash" : "b", "baz" : "c"}}</code>.</p>