zsh: fzf-completion: use noshwordsplit local option

This also fixes the completion causing a bell / flickering in case
"shwordsplit" was not set, because then the function would return false.

zsh completion: use \grep to skip any alias

Related: #373

Close #412

Close #413

[bash-completion] Add nvim to f_cmds

Not relying on exit status for CTRL-R

Patch submitted by @robinro and @JackDanger
Close #403 #242 #241 #203

In the case that fzf-tmux returns a user-selected result but with a
non-zero exit status (which can happen if a function inside $PS1 returns
non-zero) this allows CTRL-R to continue working as expected.

Addresses #203 (Tranquility's comment)

This prevents fzf from missing SIGWINCH during startup which
occasionally happens with fzf-tmux

Close #400

/cc @xconstruct

Close #392

  usage: ./install [OPTIONS]

      --help               Show this message
      --bin                Download fzf binary only
      --all                Download fzf binary and update configuration files
                           to enable key bindings and fuzzy completion
      --[no-]key-bindings  Enable/disable key bindings (CTRL-T, CTRL-R, ALT-C)
      --[no-]completion    Enable/disable fuzzy completion (bash & zsh)
      --[no-]update-rc     Whether or not to update shell configuration files

Close #376

Trigger netrw autocommand when opening directory