Additional args to be passed to `httr::GET()`

Hi Hadley,

this is my first pull request ever - so please don't bite ;-)

## Reason for modification
I needed to pass along altered user agent information for the underlying `GET` request in `html()`

## Actual modification
In order to avoid this

```
require("httr")
content(GET(<url>,  user_agent("Mozilla/5.0")))
```

I modified `html()`: it now accepts `...` to pass additional arguments along to `httr::GET()` for this to work

```
html(<url>, encoding = NULL, user_agent("Mozilla/5.0")
```

As making use of `...` involves the need to explicitly specify `encoding = NULL` in calls to `html()` (unless making `...` the second argument in `html()`), you might want to go with the second (commented-out) implementation of `html()` featuring an actual formal argument `httr_config` for the things to pass to `httr::GET()`.

## Unit test
I also added a short unit test for a German website that throws a `403` error for the default user agent.

Best regards and thanks for the cool package!
Okay, I'll change that. Just didn't want to break your current order as this might break existing code that relies on `rvest`.