Add. `zthreads.run` and `zthreads.fork` functions.
`run` function just uses runstring/runfile functions to work.

```lua
local thread, skt = zthreads.fork(zmq:context(), [[
  local skt = arg[1]
  skt:send("hello")
  print("thread:", (skt:recv()))
]])
thread:start()
print("  main:", (skt:recv()))
skt:send("world")
```