Add option for sagetex

[atticus-sullivan]

sage aims to be an alternative to commercial programs such as Maple, Matlab, and Mathematica. sagetex is a package to include sage commands/expressions in TeX documents and doing calculations/plotting thereby. Using sagetex requires run tex -> sage -> tex. As this workflow is pretty similar to the one of biber/makeglossaries, we can implement it quite the same way. So this just checks if sage should be run (file with ext = sage found in filelist has changed) and if so, run sage.

To be discussed:

• In cluttex.lua for makeglossaries and biber you always touch the supposed output file. I'm not quite sure why you do this, so I just didn't copy this. Maybe you can explain why you're doing this and maybe we should do this for sagetex as well.
• In addition you insert the supposed output file to the filelist. I'm still not quite sure what the effects of this are, so this time as well I just didn't copy it (even tough it most likely is important).

So again @minoki, what are your thoughts on this? (made this a draft again to first discuss these points)

[atticus-sullivan]

Anything you can tell about the two points I mention?