Local config, binders and formatters

This pull-request adds support for overriding any of the global `config`, `binders` and `formatters` objects on a per view basis by shallow merging in any local options with the global ones and storing cloned versions of them on the view.

```coffeescript
# Pass in any global overrides as a third argument
# to rivets.bind.
view = rivets.bind el, {item: item},
  config: …
  binders: …
  formatters: …

# Now there are local versions of those options
# directly on the view that can be accessed from the
# binding instances.
view.config
view.formatters
view.binders
```