fix typo in compile-nonterminal

The produced named lambda should be called COMPILED-NONTERMINAL
instead of COMPILE-NONTERMINAL
added syntax (function FUNCTION-NAME) for function terminals

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

* The system version has been bumped from 0.11 to 0.12 and a feature
  :ESRAP.FUNCTION-TERMINALS is pushed onto *FEATURES* when the system
  is loaded

* 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' and Kilian's ideas and feedback.