Add --black for terminals incapable of use_default_colors

See the discussion in #18.

Use --black option to use black background regardless of the default
background color of the terminal. Also, this option can be used to fix
rendering issues on terminals that don't support use_default_colors (man
3 default_colors). Depending on the terminal, use_default_colors may or
may not succeed, but the Ruby version of it always returns nil, it's
currently not possible to automatically enable this option.

README.md

@@ -59,6 +59,7 @@ usage: fzf [options]
     +i                   Case-sensitive match
     +c, --no-color       Disable colors
     +2, --no-256         Disable 256-color
+        --black          Use black background
         --no-mouse       Disable mouse
 
   Environment variables