Adds a Generic Engine

[TianyiShi2001]

A generic engine that make it possible to run any language using custom commands.

This is just a quick-and-dirty implementation (from conception to implementation in one night). If you find this interesting, please post your comments here so I can go ahead and add more features/make it more robust.

[yihui]

I definitely like the idea of providing a generic engine to run arbitrary commands. There are a couple of issues with the current implementation, but I don't have time at the moment to mention all of them. I'll revisit this PR in a few weeks. The main problem in my eyes is the templating syntax {}, which may not be robust or flexible. I'd probably consider using sprintf() and %s. Thank you!

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[yihui]

Hi @TianyiShi2001, I want to let you know that we have just provided a generic engine for executing commands, and you can find the documentation and examples at https://github.com/yihui/knitr-examples/blob/master/124-exec-engine.Rmd. Please feel free to let us know if you have any suggestions or comments. Many thanks for your original idea!