adds a parameter to running

[Tieske]

Allowing to patch only problem locations instead of monkey patching coroutine.running. But retains full compatibility with coroutine.running behaviour (so still can monkey patch).

Adds to #7

Example:

local cocall = require("coxpcall")

local myfunc = function(coro)
   coro = cocall.running(coro)   -- convert to original coroutine
   -- here do what needs to be done...
end