add a runtime for cli execution

[Tieske]

adds a copas cli command to directly run copas based code. Example:

#!/usr/bin/env copas

local count = 0
copas.timer.new {
  delay = 1,
  recurring = true,
  callback = function(self)
    count = count + 1
    print('hello world ' .. count)
    if count >= 5 then
      self:cancel()
    end
  end
}

Copas itself is exposed as a global. And the loop is automatically started.

The submodules are now lazy-loaded upon first use and are accessible through the copas module.