luacov: reporter: select files/modules on command line

[blueyed]

luacov --help says:

Examples:
luacov foo/bar

  This will report only on modules in the foo/bar subtree.

But this appears to have no effect.

The code appears to use only the same file_included method like the runner:

https://github.com/keplerproject/luacov/blob/5a2900ea06222c69777aa3b4b47d5ef7017c72d0/src/luacov/reporter.lua#L37-L52

btw: it would also be useful to output to stdout (without having to edit
.luacov, if specifying - / stdout works there at all).