Add concatenation (#15)

* Add concatenation

* Update documentation

* Match immutable concat behavior

* Fix incorrect spelling

* Update documentation

* Update test cases and better concatenation of complicated objects

* Update README.md

Co-Authored-By: Abbondanzo <[email protected]>

* Use the correct type for update parameter

* Updaet documentation

* Removed indexedSeq protocol and internal implementation

* Target indexables only

* Update test cases

* Update test names and documentation

* Add instance checking

* Update src/__tests__/concat-test.js

Co-Authored-By: Kevin Johnson <[email protected]>

* Update src/__tests__/concat-test.js

Co-Authored-By: Kevin Johnson <[email protected]>

* Update src/__tests__/concat-test.js

Co-Authored-By: Kevin Johnson <[email protected]>

* Include undefined in test suite