Expert downloads

In reference to Issue [#172](https://github.com/jazzido/tabula/issues/172): "dimensions of this selection in a pre-written command for tabula-extractor": I started working on this because I want to use Tabula as an unofficial frontend for other pdf extraction methods like OCR, see https://github.com/opensecrets/OCRToolkit.  

I added "Tabula-Extractor Script" and "Bounding Boxes" under "CSV" and "TSV" in the download data dropdown.  The first will output:

```sh
tabula -a 150.7066381156317,385.5032119914347,331.9914346895075,601.7344753747324 -p 1 "$1"
tabula -a 337.45182012847965,16.38115631691649,559.1434689507495,532.9336188436831 -p 1 "$1"
```
Which, as a shell script, will take the name of a pdf and extract the same or a different file in the same way.  

And "Bounding Boxes" is something like this:

```json 

{"x1":385.5032119914347,"x2":601.7344753747324,"y1":150.7066381156317,"y2":331.9914346895075,"page":1}
{"x1":16.38115631691649,"x2":532.9336188436831,"y1":337.45182012847965,"y2":559.1434689507495,"page":1}
```

Both very useful for my purposes, thought I'd send these back.