Support Unicode input

[porras]

Output was already fine. So I think this is all that's needed to support Unicode. It is testable with example/input.cr. I also used this example to test, that in the end I didn't include in the PR because it's so similar to the one there's already in the repository:

require "./../src/ncurses"

NCurses.start
NCurses.cbreak
NCurses.no_echo
NCurses.no_timeout
NCurses.keypad true

NCurses.print "Type any char (including Unicode like Ñ or 💃)", 2, 10
NCurses.print "Q or 🚪 to exit", 3, 15
NCurses.move 5, 15

NCurses.refresh

NCurses.get_char do |ch|
  break if ch.is_a?(Char) && "Q🚪".includes?(ch)
  NCurses.print(ch.inspect, 5, 15)
  NCurses.clear_to_eol
  NCurses.refresh
end

NCurses.end

Of course, it would be possible to add this method with a different name instead of replacing it, but I thought this would be ok because it's backwards compatible (obviously works with ASCII input) and it just makes sense in a language like Crystal that supports Unicode by default. Let me know if you think that having an additional get_char_w or something like that would be better.

This is the only method currently implemented that would need to change if I'm not mistaken. I thought of adding wget_wstr, but since wgettstr is not implemented, I left it so.

Fixes #13.