use package.json options + custom precompiler/compiler options

In response to issue #24. 

This allows the user to specify the following options in their `package.json` (the way browserify is doing it now):

```json
{
  "hbsfy": {
    "extensions": ["html"],
    "precompiler": "my-precompiler-module",
    "compiler": "require('my-compiler') or window.myCompiler"
  }
}
```

`hbsfy.configure` overrides any configurations in the `package.json`, and all options are optional (hbsfy works as expected with no options).