Support branch code coverage

[ligurio]

Is it possible to add support of branch coverage to luacov?
Right now luacov supports only coverage by statements and coverage by paths (#49) and branch coverage would be interesting too.

[hishamhm]

Unfortunately, I don't think it's viable. We collect information based on what the Lua debug library provides, and it is not fine-grained at the branch level. Its finest grain is at VM instruction level, but would try to map each single instruction back to the Lua source code branches, and that would be implementation-dependent based on the Lua compiler code generation, which can change on each minor release; it doesn't sound very feasible.

I'd love to be proven wrong by a PR though! :)