Better response file support

[gittup]

Currently tup supports response files by using a group to replace the filenames. It would be better if it could also support response files in more general cases of just long command-lines that aren't the result of many input files.

[monsanto]

I was unable to make use of the current response file support, as (to my knowledge) you can't add files to groups unless they are generated by a rule.

It would be really useful if response files could at least be created for %f and %o, maybe by adding a character after the % like the quoting mechanisms.

Alternatively, an easy way to make this more general & Lua-friendly is to add a stdin=<string> option to tup.definerule.