- Unset multi-select option with +m

- Added examples: fbr and fco
- Always use local variables

This is automatically set in install script. It may only cause unnecessary
confusion.

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.

e.g. urxvt

Since the version 0.7.0, fzf internally used Curses.getch() call to take user
input, which allowed it to support mouse input as well. However it has turned
out that Curses.getch() has introduced glitches that cannot be easily handled
(e.g. Try resize the terminal). So I finally decided that it's not worth the
trouble and drop the mouse support.

This commit adds --filter option so that fzf can be used as a simple unix
filter instead of being an interactive fuzzy finder.