Add support for custom function registration in Java API

[mbali]

The Clojure API makes it simple to register custom function definitions by defining a new dispatch for the call-fn multimethod.

On Java side we do not have such an easy way to achieve this, without reimplementing API.render. I had multiple approaches in my mind:

1. Registering one or more static function providers in FunctionEvaluator. This would change the global state, so it can have unexpected consequences.
2. Amending the FunctionEvaluator instance during the render phase. I went with this approach.

I added a new API.render overload which accepts custom function implementations as its final parameter.
I also added test cases. NB: It looks like lein test does not run the jUnit tests in java-src.

[codecov]

Codecov Report

Merging #101 (8bbb98d) into master (000ae43) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #101   +/-   ##
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  Coverage   93.00%   93.00%           
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  Files          25       25           
  Lines        1873     1873           
  Branches      107      107           
=======================================
  Hits         1742     1742           
  Misses         24       24           
  Partials      107      107           

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