pass pkg.__dirname to transform functions

As we cannot write `__dirname` to package.json, transforms which require directories must be given relative paths.

package.json

``` json
{
"browserify": {
"transform": [
["transform1", {"dirname": "relative/path"} ]
]
}
}
```

However, there is no way to resolve absolute paths. `process.cwd()` varies by where node process is created.

Pass `pkg.__dirname` to transform functions and we can calculate absolute paths from where the package.json file is located.