#20 Output warning messages in qalc through stderr

[Cousteau]

Warning messages in the `qalc` command line program should be output to stderr rather than stdout. Similarly for error messages (although in that case the stdout should contain "[error]" or explicitly notify there was an error). This way it would be easier to redirect qalc's output to other programs.

As an extra feature, there could be a -q option to suppress warning messages (although this could be done by doing `qalc 'expression' 2>/dev/null`) and a -v one for printing it as stdout rather than stderr as it does right now (although this could be done by doing `qalc 'expression' 2>&1`).