Parse files listed via STDIN

wdd_select can now parse more than one file and the list of files can be
passed via STDIN to allow for nicer piping.

Example:

```bash
$ find data_samples -name "*.html" | wdd_select "head, body"
data_samples/forecast_io.html,head
data_samples/forecast_io.html,body
data_samples/jimsmarketingblog_com.html,head
data_samples/jimsmarketingblog_com.html,body
```