Implements primitive 'readline' inside CompletionEngine

Issue: #52
Adds a dumb version of CompletionEngine without libreadline

Issue: #52
Adds handling of READLINE option in cmake

Now usage of readline library may be controlled
by passing a -DREADLINE=(ON|OFF) parameter to cmake.

If parameter is passed library usage depends on value.
If parameter is omitted it defaults to ON.

Issue: #52
Merge branch 'feature/52/optional_readline' into develop

* feature/52/optional_readline:
  Adds handling of READLINE option in cmake
  Adds a dumb version of CompletionEngine without libreadline
  Implements primitive 'readline' inside CompletionEngine

Conflicts:
	CMakeLists.txt
	src/CompletionEngine.cpp
	src/vm.cpp

Issue: #52