Sequential commands

I'm attempting to add support for a knitr > latex > pdf work flow, similar to what I have used in textmate in the past. For all of these additions I will (of course) submit pull requests, once I know I've got everything working ok.

I have successfully setup commands for both knitr and latex separately:

```
Knitr:
"File Based":
command: "Rscript"
args: (context) -> ['-e', 'library(knitr); knit("'+context.filepath+'", output="'+context.filepath.replace('.Rnw','.tex')+'")']

LaTeX:
"File Based":
command: "texi2pdf"
args: (context) -> ['-c', context.filepath, '-o', context.filepath.replace('.tex','.pdf')]
```

But what I would really like to do is have a way to run the knitr command and then the latex command in sequence with one command. Does script have any sort of support for sequences of commands like this? If not, does anyone have ideas on how that might be accomplished?