Refactor shell extensions

- Use symlinks instead of generating the full content
- Update fish_user_paths and remove ~/.config/fish/functions/fzf.fish
- Create wrapper script for fzf when Ruby version and use it instead of
  exported function not to break fzf-tmux

When 1 is given, 50% of the screen width or height will be used as the
default size of the pane.

As they're made synchronous with the use of fzf-tmux script

[fzf-tmux] Allow opening fzf on any position (up/down/left/right)

The previous -w and -h will be synonyms for -r and -d respectively.

fzf-tmux -w -q q
fzf-tmux -w -- -q q

Add fzf-tmux script

This avoids the problem in which :FZF command silently fails when fzf
executable cannot be found in $PATH of the hosting tmux server.

/cc @Tranquility

Related: #135

0.9.4

When fzf works in filtering mode (--filter) and sorting is disabled
(--no-sort), there's no need to block until input is complete. This
commit makes fzf print the matches on-the-fly when the following
condition is met:

    --filter FILTER --no-sort [--no-tac --no-sync]

or simply:

    -f FILTER +s

This removes unnecessary delay in use cases like the following:

    fzf -f xxx +s | head -5

However, in this case, fzf processes the input lines sequentially, so it
cannot utilize multiple cores, which makes it slightly slower than the
previous mode of execution where filtering is done in parallel after the
entire input is loaded. If the user is concerned about the performance
problem, one can add --sync option to re-enable buffering.

- Key bindings for bash, zsh, and fish
- Fuzzy completion for bash (file, dir, process)

DISCLAIMER: This is a backward incompatible change