Add machine parsable output to list available counters

[UsualSpec]

This is probably a very niece feature, but we would largely benefit from it.

The -l option provides human readable outputs for counters, aliases,... However, it would be great to have JSON output or similar such that other programs can easily query the program.
In our project, we want to verify and later on tell the user which counters are available in a Web UI. Therefore, in nsg-ethz@6683503 we introduced manual JSON output with a new -L option.

I'll open a PR once this has settled.