Evaluate the allowed characters for perfdata label

[RincewindsHat]

At least the combination Icinga 2 + InfluxDB + Grafana seems to handle Unicode characters well, so maybe we should allow those.
Someone has to test that for Graphite and RRD backends.

[martialblog]

@RincewindsHat and I think we can loosen the regex a bit and only remove the most critical characters (quotes)

[martialblog]

@RincewindsHat This should also be in v0.5.0