Add SageMath extension

[akerbos]

Spontaneous addition after reading TUGBoat 2016.1 p25ff

[reitzig]

TODO

• Detect warnings in main log:

  LaTeX Warning: Reference `@sageinline0' on page 1 undefined on input line 10.
  
  Package sagetex Warning: Graphics file sage-plots-for-sagetex_errortest.tex/plo
  t-0.pdf on page 1 does not exist. Plot command is on input line 14.
  
  LaTeX Warning: Reference `@sageinline1' on page 1 undefined on input line 16.
  
  Package sagetex Warning: Graphics file sage-plots-for-sagetex_errortest.tex/plo
  t-1.pdf on page 1 does not exist. Plot command is on input line 17.
  
  *********************************************************************
  
  Package sagetex Warning: there were undefined Sage formulas and/or plots.
  Run Sage on sagetex_errortest.sagetex.sage, and then run LaTeX on sagetex_error 
  test.tex again.

• Log messages from SageTex are swallowed for some reason (parsing works).