Normalize relative paths in babel.extends, plugins, and presets to package.json directory

[jamestalmage]

Probably best handled along with #700.

Assuming following config in package.json:

{
  "ava": {
    "babel": {
       "extends": "./some-config.json"
     }
  }
}

It will work as long as process.cwd() is the same as package.json.

cli.js should normalize the path to package.json (api.js does not know anything about package.json).

This should be part of the config normalizing discussed in #700

[novemberborn]

This will be done as part of RFC001.