Lazy load nvm

nvm takes on average half of a second to load,
which is more than whole prezto takes to load.
This can be noticed when you open a new shell.
To avoid this, we are creating placeholder function for nvm, node,
and all the node packages previously installed in the system to only
load nvm when it is needed.

This code is based on the scripts:
* https://www.reddit.com/r/node/comments/4tg5jg/lazy_load_nvm_for_faster_shell_start/d5ib9fs
* http://broken-by.me/lazy-load-nvm/
* https://github.com/creationix/nvm/issues/781#issuecomment-236350067

modules/node/init.zsh

@@ -6,13 +6,43 @@
 #   Zeh Rizzatti <zehrizzatti@gmail.com>
 #
 
+lazy_load_nvm() {
+  NVM_DIR=$1
+  # Skip adding binaries if there is no node version installed yet
+  if [ -d $NVM_DIR/versions/node ]; then
+    NODE_GLOBALS=(`find $NVM_DIR/versions/node -maxdepth 3 \( -type l -o -type f \) -wholename '*/bin/*' | xargs -n1 basename | sort | uniq`)
+  fi
+  NODE_GLOBALS+=("nvm")
+
+  load_nvm () {
+    # Unset placeholder functions
+    for cmd in "${NODE_GLOBALS[@]}"; do unset -f ${cmd} &>/dev/null; done
+
+    # Load NVM
+    [ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"
+
+    # (Optional) Set the version of node to use from ~/.nvmrc if available
+    nvm use 2> /dev/null 1>&2 || true
+
+    # Do not reload nvm again
+    export NVM_LOADED=1
+  }
+
+  for cmd in "${NODE_GLOBALS[@]}"; do
+    # Skip defining the function if the binary is already in the PATH
+    if ! which ${cmd} &>/dev/null; then
+      eval "${cmd}() { unset -f ${cmd} &>/dev/null; [ -z \${NVM_LOADED+x} ] && load_nvm; ${cmd} \$@; }"
+    fi
+  done
+}
+
 # Load manually installed NVM into the shell session.
 if [[ -s "$HOME/.nvm/nvm.sh" ]]; then
-  source "$HOME/.nvm/nvm.sh"
+  lazy_load_nvm "$HOME/.nvm"
 
 # Load package manager installed NVM into the shell session.
 elif (( $+commands[brew] )) && [[ -d "$(brew --prefix nvm 2>/dev/null)" ]]; then
-  source $(brew --prefix nvm)/nvm.sh
+  lazy_load_nvm "$(brew --prefix nvm)"
 
 # Return if requirements are not found.
 elif (( ! $+commands[node] )); then