Add output format and filter and extend online help

[pbiering]

Hi, thank you first for your script, was able to use this on QNAP with InfluxDB and Telegraf!

This PR contains "outputformat" and "outputfilter" option to avoid requesting additional external wrapper scripts like mentioned here: https://github.com/jhubig/FritzBoxShell/wiki/Example-use-cases:-Telegraf

It was implanted by "minimal invasion" to avoid major changes in the code on several parts because of the current structure. In case "outputformat" is active, it's simply calling itself again after parsing the options and reformats the output.

Currently supported: "influx" and "graphite"-like output

It also extends the online help.