Backtrace cleaner

Related #798

I added the backtrace cleaner object. I felt that discard was a better name than clean given that we now have a second class of line (include) to compare it to. Clean and include seemed less intuitive than discard and include to me. (I did not change the names of the configuration options).

This causes two of the specs to fail on my machine, it looks like it's due to the fact the nature of what lines get included have changed, and this causes the pre-formatted html to be different. Would the best change to make these specs have an empty include patterns configuration option?