Added luacheck.flow

luacheck.flow builds program flow graphs.
Currently unplugged and only tested manually.
See https://gist.github.com/mpeterv/d1741738876e9923e77c
Requires links between gotos and labels and list of closures.
   Note: metalua actually crashes on gotos. Fork it?
Will be used for flow-aware unused value detection in 0.6.0.
And for variable resolution and dead code detection later.