Add -M flag to map non-numeric perfdata values

[martialblog]

This PR adds a new -M flag that can be used to map non-numeric values to numeric values:

check_http_json.py -H host.internal --path example-data.json -e 'service.status' -m 'service.status'
OK: 'service.status'=Up  Status OK. |'service.requests'=Up

check_http_json.py -H host.internal --path example-data.json -e 'service.status' -m 'service.status' -M Up=1
OK: 'service.status'=1  Status OK. |'service.requests'=1

Fixes #108