#1 Add "Readline" support to -i (interactive) mode

[defunctc++]

It would be a very nice feature to have variable and operator tab-completion and support for the arrow keys while editing RPN strings in interactive mode (-i).

This can be achieved by compiling with support for Readline, which is a common *nix commandline user-interaction library.

Tab completion would make editing a much more streamlined experience, it could complete operator names (atomic and custom) and variable and constant names, to accelerate input and just generally increase the awesomeness of the program.