Why

Sometimes we want to share the same .grenrc.js among many projects, and manage it using npm package, just like what prettier did.
This is a job finishing pr #230 .

How

1. treat gren as npm package name if it does not start with 'http'
2. use npm's api to get the latest package version
   a. use require(child_process).spawnSync so that async operation could act like sync operation in main process
3. use jsdelivr to get the config file

Test

• Every broken test instances have been changed or removed;
• We have made this change in this fork, and using it among our projects such as el-data-table, just works