Support CRAN packages

This allows to automatically bump R packages. I only found unofficial and undocumented APIs, so parsing `https://cran.r-project.org/package=foo` seems the only option. The version is in two `<td>` elements, e.g.

```
<td>Version:</td>
<td>1.1.4</td>
```
for `https://cran.r-project.org/package=abctools`.