performance: pnmtops to temp file
performance: pnmtops to temp file

For some reason when piping the output of pnmtops directly to ps2pdf,
conversion is very slow (especially for color scans).

When writing to an intermediate temporary file, conversion is fast.

Before:

________________________________________________________
Executed in   19.20 secs   fish           external
  usr time    4.01 secs    0.32 millis    4.01 secs
  sys time   20.63 secs    5.30 millis   20.62 secs

After:

________________________________________________________
Executed in  368.06 millis    fish           external
   usr time  378.06 millis    0.00 millis  378.06 millis
   sys time  100.00 millis    2.79 millis   97.21 millis

An improvement of over 50 times!

This should resolve #19.

rocketraman/sane-scan-pdf