UTF8 Support

[ericcurtin]

This wasn't thoroughly tested

[ericcurtin]

@yhirose you are probably like... Not again! But if you want to test and implement this again here in c++17, feel free.

[ericcurtin]

And I'm not against huge changes to the codebase in it's early days :) (lets say we decide, no cout, no exceptions)

[yhirose]

@ericcurtin the original linenoise supports UTF-8! So there is no reason for me to work on this. :)
antirez/linenoise#187

[ericcurtin]

Fair enough... :) The biggest difference between the two now, would be readline.cpp has Windows support and linenoise doesn't