Add JSON output or similar

[giddel]

It would be nice, if the read output can be formatted as json, yaml, csv or anything else which can easily parsed by other code (e.g. shell scripts)
For example instead of:

$ mbpoll 192.168.1.148 -r 40068 -q -1 -c4
-- Polling slave 1...
[40068]:    7898
[40069]:    0
[40070]:    65524 (-12)
[40071]:    65535 (-1)

Something like this:

$ mbpoll 192.168.1.148 -r 40068 -q -1 -c4  -o json
{
    "slave": 1,
    "registers": [
        {"40068": 7898 },
        {"40069": 0},
        {"40070": "65524 (-12)"},
        {"40071": "65535 (-1)"}
    ]
}