Support OnHttpRequest Lua callback

If your redbean `/.init.lua` file defines a global callable named
`OnHttpRequest` then redbean will delegate all serving control to
your function. You may then restore the default serving paths, by
calling the new `Route()`, `RouteHost()`, and `RoutePath()` APIs.

Closes #150

tool/net/demo/hello.lua

@@ -0,0 +1 @@
+Write('hello world\r\n')