Abstract away stdin/stdout constants

[brjoha]

I think it would be helpful to abstract away the direct use of stdin/stdout constants into variables (that default to stdin/stdout) such that a pty could be swapped in, allowing for a socket input source.

I think it would also be helpful to allow for an event loop environment via a character reader API (reference http://www.delorie.com/gnu/docs/readline/rlman_41.html).