add silent to WECHATY_LOG

src/npmlog-env.js

 const log = require('npmlog')
 
-const level = process.env.WECHATY_DEBUG || process.env.WECHATY_LOG
-const levelRegexStr = 'silly|verbose|info|warn|error'
+const level = process.env.WECHATY_LOG
+const levelRegexStr = 'silly|verbose|info|warn|error|silent'
 const levelRegex = new RegExp(levelRegexStr, 'i')
 if (levelRegex.test(level)) {
   log.level = level.toLowerCase()
-  log.verbose('Npmlog', 'WECHATY_DEBUG set level to %s', level)
+  log.verbose('NpmLog', 'WECHATY_LOG set level to %s', level)
 }
 else if (level){
-  log.warn('Npmlog', 'env WECHATY_DEBUG(%s) must be one of silly|verbose|info|warn|error', level)
+  log.warn('NpmLog', 'env WECHATY_LOG(%s) must be one of silly|verbose|info|warn|error|silent', level)
 }
 
 module.exports = log