New command :pprint for outputting paper/slide bits

The command:

:pprint fmt width n

outputs the definition of name n for format FMT, with a screen width of
WIDTH columns.

:pprint fmt width tm

outputs the term TM.

The allowed formats are "html" and "latex".
Merge pull request #1547 from david-christiansen/pprint-command

New command :pprint for outputting paper/slide bits