add links to 'directories' field of package.json and 'bin' field

Before, if there is a directory called 'main', it will be treated as 'main' field. For example:
```
"directories": {
   "main": "./main",
   "bin": "./bin"
 },
 "bin": "./index.js",
 "main": "index.js",
```

The html structure of `"main": "index.js"` and `"main": "./main",` are the same. So with 'bin' field and 'bin' directory.

With adding links to 'directories' field before adding links to 'main' and 'bin' fields, we can distingush directories from the fields of the same name.