nodejs默認源在國外，訪問速度慢。可以手動修改源，但要每次都修改。
安裝 nrm，指定源路徑，不需要每次修改
npm install nrm -g
nrm ls
* npm -------- https://registry.npmjs.org/
yarn ------- https://registry.yarnpkg.com/
cnpm ------- http://r.cnpmjs.org/
taobao ----- https://registry.npm.taobao.org/
nj --------- https://registry.nodejitsu.com/
npmMirror -- https://skimdb.npmjs.com/registry/
edunpm ----- http://registry.enpmjs.org/
nrm use taobao