Improved `read_hxml`.

[andyli]

This is quite a big revision of the read_hxml function. Hopefully it wouldn't break anything... Please review and test with your projects. ;)

Here is the list of improvements:

• Correctly handle nested hxml files.
• Recognise haxe class being passed without -main.
• Gracefully handle unknown compiler arguments - show warning status message and then add them anyway.
• Use l.split(" ", 1) instead of (l.split(" ") + join).

[nadako]

Hey @andyli!

It seems that there should be a bit more hackery when parsing args after --run, because everything after --run is consumed and used as --run args (and thus these args should follow the --run arg).

For example:

--run Build
arg1
arg2

Currently this will put arg1, arg2 before --run resulting to invalid command line.

[andyli]

Good catch! Will go to fix it and make a PR!