updateFile and updateFileSync support

[manidlou]

Please let me know if you think it is reasonable to add these two functions or no need for them :smile:

• updateFile(filepath, obj, cb)
• updateFileSync(filepath, obj)

They read the file, update the file contents with obj using something like Object.assign(), then write back the updated contents to the same file.

[RyanZim]

Related/duplicate of https://github.com/jprichardson/node-jsonfile/issues/52.