Replace cmdline package with cli (v2) package

[mcktr]

This drops the usage of the cmdline library and uses the cli (v2) library.

fixes #73

[mcktr]

TODO:

• [x] fix version output
• [x] fix description output
• [x] fix help output; include available methods

[mcktr]

go run cmd/check_fritz/*.go --help
NAME:

  check_fritz - Check plugin to monitor a Fritz!Box

USAGE:
   check_fritz [options...]

OPTIONS:

  --hostname value, -H value  Specifies the hostname. (default: fritz.box)
  --port value, -P value      Specifies the SSL port. (default: 49443)
  --username value, -u value  Specifies the username. (default: dslf-config)
  --password value, -p value  Specifies the password.
  --method value, -m value    Specifies the check method. (default: connection_status)
  --ain value, -a value       Specifies the AIN for smart devices.
  --timeout value, -t value   Specifies the timeout for requests. (default: 90)
  --modelgroup value          Specifies the Fritz!Bpx model group (DSL or Cable). (default: DSL)
  --warning value, -w value   Specifies the warning threshold. (default: 0)
  --critical value, -c value  Specifies the critical threshold. (default: 0)
  --help, -h                  show help (default: false)
  --version, -V               print the version (default: false)

METHODS:
  connection_status       WAN connection status,
  connection_uptime       WAN connection uptime (in seconds),
  device_uptime           device uptime (in seconds),
  device_update           update state,
  downstream_max          maximum downstream,
  upstream_max            maximum downstream,
  downstream_current      current downstream,
  upstream_current        current upstream,
  downstream_usage        current downstream usage,
  upstream_usage          current upstream usage,
  smart_heattemperatur    current temperature of a a radiator thermostat (requires AIN),
  smart_socketpower       current power consumption of a socket switch (requires AIN),
  smart_status            current smart device status (requires AIN)