Pre commands inherit stderr from main process

[gramosg]

This has two main benefits:

1. Errors during preprocessing are now visible (not on the slides
   themselves but back on the console after exiting), which helps with
   debugging.
2. Subprocesses are able to do fine-grained formatting, as terminfo
   commands now work properly. For example, tput cols and tput lines
   now return the correct terminal size instead of the defalt 80x24.