Allow threshold to be applied on a per file basis

[k]

This is a feature request and pool for ideas.

I can cannot achieve 100% coverage for the reasons explained here, but I want to make sure as the codebase grows that the amount of untyped code will not grow.

One way to limit this is to enforce coverage on a per-file basis. Theory being e.g. 95% on each file is better then 99% overall.

There would also need to be an additional option to specify a coverage map which would describe override thresholds for files that need it (for example, files that use exceptions from catch.

Thoughts?