os.mkdir does not expand ~ to $HOME

I've just came across this behaviour and not sure if we'd like to do something about it.

Basically if you pass a path containing a tilde to some of the OS directory functions they will not expand it to the user's home directory, which I was expecting to work since it seems to be supported in other places in Premake.

Should we fix it or just document these limitations of the API on the wiki?