Add support for saving from an HTML path

It'd be useful to be able to create webarchive files from HTML files on disk, but currently that fails:

```
$ ./save_safari_webarchive index.html
Usage: ./save_safari_webarchive <URL> <OUTPUT_PATH>

$ ./save_safari_webarchive /Users/alexwlchan/repos/safari-webarchiver/index.html
Usage: ./save_safari_webarchive <URL> <OUTPUT_PATH>
```