Allow switching the "primary" coverage criterion

[PragTob]

It'd be nice if we were able to switch what you want to see as the primary coverage criterion. Right now simplecov still always treats line coverage as the main thing that it displays big and bold.

If we could do something like:

SimpleCov.start do
  enable_coverage :branch
  primary_coverage :branch
end

It'd be great if these exchanged places and branch coverages would be considered the main and default coverage type.

Meaning not only that it'd be displayed in the HTML formatter and console output first and foremost, but also that if we just did minimum_coverage 90 it'd mean a minimum of 90% branch coverage instead of line coverage (which it currently hard defaults to).