Wrap nvm install to add support for rc/nightly

zsh-nvm.plugin.zsh

@@ -61,6 +61,9 @@ _zsh_nvm_load() {
         _zsh_nvm_nvm "$@"
         export NVM_AUTO_USE_ACTIVE=false
         ;;
+      'install')
+        _zsh_nvm_install_wrapper "$@"
+        ;;
       *)
         _zsh_nvm_nvm "$@"
         ;;
@@ -172,6 +175,20 @@ _zsh_nvm_auto_use() {
   fi
 }
 
+_zsh_nvm_install_wrapper() {
+  case $2 in
+    'rc')
+      NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/rc/ nvm install node
+      ;;
+    'nightly')
+      NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/nightly/ nvm install node
+      ;;
+    *)
+      _zsh_nvm_nvm "$@"
+      ;;
+  esac
+}
+
 # Don't init anything if this is true (debug/testing only)
 if [[ "$ZSH_NVM_NO_LOAD" != true ]]; then