Add perfdata thresholds to the output if present

Depending on the metric in question, perfdata often arrives with
specified thresholds for warn/critical values, and min/max possible
values as well. Those are nice to keep.
Tidy code according to hlint suggestions

`second` is the best thing evar, that is all.
Merge branch 'master' into barney-add-perfdata-thresholds
Clean up code, remove evil Maybes and not-quite-filters
Merge pull request #24 from anchor/barney-add-perfdata-thresholds

Looks good, merging.