feat: export MFS root key

So that GC can avoid collecting your MFS https://github.com/ipfs/js-ipfs/pull/2022/files#r303383848

Author: Alan Shaw

src/index.js

@@ -4,12 +4,14 @@ const cli = require('./cli')
 const core = require('./core')
 const http = require('./http')
 const {
-  FILE_TYPES
+  FILE_TYPES,
+  MFS_ROOT_KEY
 } = require('./core/utils/constants')
 
 module.exports = {
   cli,
   core,
   http,
-  FILE_TYPES
+  FILE_TYPES,
+  MFS_ROOT_KEY
 }