Prometheus is standardizing on kingping for flag parsing, as discussed
in https://github.com/prometheus/prometheus/issues/2455. Many exporters
are following this decision so most of the Prometheus ecosystem uses
kingpin.

As a consequence of this change, flags now require double dashes.
`-web.listen-address` becomes `--web.listen-address` for example.

Usage before:

```
Usage of ./dovecot_exporter:
  -dovecot.socket-path string
    	Path under which to expose metrics. (default "/var/run/dovecot/stats")
  -web.listen-address string
    	Address to listen on for web interface and telemetry. (default ":9166")
  -web.telemetry-path string
    	Path under which to expose metrics. (default "/metrics")
```

And after:

```
usage: dovecot_exporter [<flags>]

Prometheus metrics exporter for Dovecot

Flags:
  --help                        Show context-sensitive help (also try --help-long and --help-man).
  --web.listen-address=":9166"  Address to listen on for web interface and telemetry.
  --web.telemetry-path="/metrics"
                                Path under which to expose metrics.
  --dovecot.socket-path="/var/run/dovecot/stats"
                                Path under which to expose metrics.
```