* Switch cmd/amtool to kingpin
* Touch-ups
* Implement long help
* Add missing short-form of --output
* Fix backwards compatibility for config file options
* Fix vendoring
* Review fixes
* Fix flag word order
* Add a new `silence import` command to amtool
This command read silences data from a query JSON output and import to
alertmanager. It allows `amtool` to be used as a backup/restore tool for
silences, i.e. #1000
Backup / export:
```
amtool silence -o json > silences.json
```
Restore / import:
```
amtool silence import silences.json
```
* Add a WaitGroup barrier
Move error channel reading to a goroutine to prevent deadlock and thus
add a WaitGroup to synchronize.
Calle Pettersson
Binh Le