Add XDG npm configuration

Add configuration to move npm to a sensible XDG basedir configuration.
Move npmrc to config, cache to cache, and global packages to data.

zsh/.zshenv

@@ -80,3 +80,9 @@ export ATOM_HOME="$XDG_DATA_HOME"/atom
 
 # Set up mplayer
 export MPLAYER_HOME="$XDG_CONFIG_HOME"/mplayer
+
+# Set up npm
+export NPM_CONFIG_USERCONFIG=$XDG_CONFIG_HOME/npm/npmrc
+if [[ -d "$XDG_DATA_HOME/npm/bin" ]]; then
+	path+="$XDG_DATA_HOME/npm/bin"
+fi