adds a plugin to log just peerIDs for nodes we connect to, to allow us to see how many unique peers a node connects to over time.
It's set up a preloaded plugin, so it would be enabled by default. We may want to extract it to a seperate repo so we can make it an optional add on.
**Usage**
```console
$ GOLOG_FILE=~/peer.log IPFS_LOGGING_FMT=json ipfs daemon
```
**output**
```json
{"level":"info","ts":"2020-02-10T13:54:26.639Z","logger":"plugin/peerlog","caller":"peerlog/peerlog.go:51","msg":"connected","peer":"QmS2H72gdrekXJggGdE9SunXPntBqdkJdkXQJjuxcH8Cbt"}
{"level":"info","ts":"2020-02-10T13:54:59.095Z","logger":"plugin/peerlog","caller":"peerlog/peerlog.go:56","msg":"disconnected","peer":"QmS2H72gdrekXJggGdE9SunXPntBqdkJdkXQJjuxcH8Cbt"}
```
License: MIT
Signed-off-by: Oli Evans <oli@tableflip.io>
