lua: allow ~ path convention for --lua

Paths passed to the --lua option now follow the convention for paths
starting with ~ documented in mpv.rst.
waf: try to fix unicode/byte string messup

Some mpv builds identify with e.g. "mpv b'0.3.3' ". The version looks
like str() was called on a Python byte string. I couldn't reproduce it
on my machine (I tried with both Python 2 and 3), so I'm not exactly
sure what's going on here, but I'm hoping this commit does fix it.