Fix #51 and another issue with omitting -w parameter

Fix for #51 and another problem: When the -w parameter was omitted, the performance data didn't contain warning/critical limits:

    | '*.xml'=11 age_oldest=11...

With this fix, it will now show up as:

    | '*.xml'=11;;20 age_oldest=11...

This also fixes specifying "~" as warning parameter which gave a similar false WARNING message as in #51 for the -a parameter.