Auto-height can now be set using description

Using objects' IDs was simple, but it has its problems - especially
the fact that IDs change for each object copy, so setting heights
for extrusion can sometimes become quite a PITA.

It is now possible to use lines like these:
ht: XXX mm
height: XXX mm

to set the object's extrusion height to XXX milimeters (where XXX is any
decimal number).
Changed according to review

Simplified the code which handles finding the auto-height data.
Merge pull request #5 from rajcze/master

Auto-height can now be set using description