add option to omit white-only entries from repl history

[averynortonsmith]

(edited first PR attempt to fix issue with branches)

I'm hoping to use this library for the repl of my language, Pointless. I wonder whether you'd consider pulling a small change I've made. My fork adds boolean param to the repl constructor that controls whether whitespace-only repl inputs are added to the repl history (set to true by default for backwards-compatibility). It's a nice option to have since most PL repls leave out blank lines.

Thanks for making this library btw -- it's just what I've been looking for!