[zsh-completion] Allow custom completion function

While in bash you can externally register custom completion functions
using `complete` command, it was not possible to do so in zsh without
changing completion.zsh as the name of the supported commands are
hard-coded within the code (See #362). With this commit, fzf-completion
of zsh will first look if `_fzf_COMMAND_completion` exists and calls the
function, so one can externally define completion functions for specific
commands.

This commit also tries to make the interface of (yet undocumented)
_fzf_list_completion helper function consistent across bash and zsh.

So the following code works both on bash and zsh.

    _fzf_pass_completion() {
      local pwdir=${PASSWORD_STORE_DIR-~/.password-store/}
      local stringsize="${#pwdir}"
      let "stringsize+=1"
      _fzf_list_completion '+m' "$@" << "EOF"
        find "$pwdir" -name "*.gpg" -print | cut -c "$stringsize"- | sed -e 's/\(.*\)\.gpg/\1/'
    EOF
    }

    # Only on bash
    complete -F _fzf_pass_completion -o default -o bashdefault pass

Note that the suggested convention and the interface are not yet final
and subject to change.

/cc @d4ndo

shell/completion.bash

@@ -195,7 +195,7 @@ _fzf_kill_completion() {
 
 _fzf_telnet_completion() {
   _fzf_list_completion '+m' "$@" << "EOF"
-  \grep -v '^\s*\(#\|$\)' /etc/hosts | \grep -Fv '0.0.0.0' | awk '{if (length($2) > 0) {print $2}}' | sort -u
+    \grep -v '^\s*\(#\|$\)' /etc/hosts | \grep -Fv '0.0.0.0' | awk '{if (length($2) > 0) {print $2}}' | sort -u
 EOF
 }
 
@@ -207,13 +207,13 @@ EOF
 
 _fzf_env_var_completion() {
   _fzf_list_completion '-m' "$@" << "EOF"
-  declare -xp | sed 's/=.*//' | sed 's/.* //'
+    declare -xp | sed 's/=.*//' | sed 's/.* //'
 EOF
 }
 
 _fzf_alias_completion() {
   _fzf_list_completion '-m' "$@" << "EOF"
-  alias | sed 's/=.*//' | sed 's/.* //'
+    alias | sed 's/=.*//' | sed 's/.* //'
 EOF
 }