Improve Fish integration script

[kidonng]

• Fish config is located at ~/.config/fish/config.fish, not ~/.config/fish/fish.conf
• Allow overriding $grc_plugin_execs, also use correct variable scope (-l for local instead of -U for universal)
• Simplify wrapper function. As said in do not colorize if output is not a terminal #81 (comment), grc will handle non-terminal scenarios and thus the extra hassle in integration script is unnecessary.