Better handling of large text files

The biggest change here is that Thumbkit truncates line length and number of lines in the text file. We just don't need to make mogrify process that much text. For now Thumbkit is pretty generous with how much text it leaves (it could truncate way more).

TODO/NOTES:

* Make truncation amount configurable.
* Make `Audio` and `Image` processors use the timeout.
* This still doesn't address the fact that mogrify hangs and does not get killed by the timeout. Fixing this mogrify bug is out of scope for thumbkit. This feature just sort of sidesteps the issue.

Honestly, I think I'll just merge this now (it fixes the major problem), and add configuration if it is requested.