allow multiple arguments to yat-config

[Peter]

I noticed that yat-config exists immediately when finding some options. --cxx for example triggers an immediate exit. I see no reason why not change that so one can issue something like

yat-config --cxx --cppflags --cxxflags

In the current implementation that would output

c++

in my configuration, while I would expect something like

c++ -pthread -g -O3

The --help and different versions of --version could probably be as is.

[Peter]

In 3800:

allow multiple arguments in yat-config. closes #911