added syntax (function FUNCTION-NAME) for function terminals

* FUNCTION-NAME is called to consume some input and return a
  production

* New example in example-function-terminals.lisp

* Added unit tests FUNCTION-TERMINALS.{PARSE-INTEGER, PARSE-A} and
  EXAMPLE-FUNCTION-TERMINALS.{INDENTED-BLOCK,READ}

* Extended README and doc/esrap.texinfo

Based on Nikodemus' idea.sketch for custom terminals