Add ZSH_NVM_NO_LOAD option

If ZSH_NVM_NO_LOAD is set to true then zsh-nvm won't actually do anything after setting up environment variables and functions. This is usefull for unit tests and debugging.

zsh-nvm.plugin.zsh

@@ -144,6 +144,9 @@ _zsh_nvm_revert() {
   fi
 }
 
+# Don't init anything if this is true (debug/testing only)
+if [[ "$ZSH_NVM_NO_LOAD" != true ]]; then
+
   # Install nvm if it isn't already installed
   [[ ! -f "$NVM_DIR/nvm.sh" ]] && _zsh_nvm_install
 
@@ -153,3 +156,5 @@ if [[ -f "$NVM_DIR/nvm.sh" ]]; then
     # Load it
     [[ "$NVM_LAZY_LOAD" == true ]] && _zsh_nvm_lazy_load || _zsh_nvm_load
   fi
+
+fi