Preserve full extension if it contains ".tar"

This fixes #580.

The new version of `file_ext` now preserves the ".tar" in ".tar.xxx":

```R
file_ext('foo.tar.gz')
# [1] "tar.gz"
```


Strings other than ".tar" aren't preserved:

```R
file_ext('foo.ar.gz')
# [1] "gz"
```