Add a function for visual diffs of revision history

Something like:

``` r
rdrop2::drop_diff("file.txt", version1 = 8, version2 = 10)`
```

The [revisions endpoint](https://www.dropbox.com/developers/core/docs#revisions) will probably be useful for this

No implementation file or test is named. Start by reviewing rdrop2's existing API functions and the linked Dropbox revisions endpoint, then determine the expected visual-diff behavior for drop_diff("file.txt", version1 = 8, version2 = 10). Done requires an agreed output and tests covering two revision versions.

Written by the indexing model from the issue text.