Add support for .buffer-static-upload.json or similar for ease of use

[djfarrelly]

Purpose

Instead of an unwieldy set of arguments to pass to the cli, we could offer a json file akin to a .babelrc.

Idea

Filename ideas:

• .buffer-static-upload.json
• `.buffer-static-upload
• .uploadrc
• .bsurc (buffer-static-upload)
• .static-upload
• Some other shorter cooler name like .suprc (static upload run command)

Format:

{
  "bucket": "my-fav-s3-bucket",
  "dir": "my-app",
  "files": ["js/*.js", "css/*.css"],
  "format": "csv",
  "output": "asset-manifest.csv"
}

Which makes the usage a much simpler ./buffer-static-upload