Describe the feature:

Sometimes, we want to use some 3rd tools (like ethotool and demicode) to do some test. On current stage we can use Command to test. But I think it's better to implement plugins so that user can define their own test method.

Describe the solution you'd like

1. a plugin (in this situtation is binary file) should follow rules as below
   1. ./plugin name will output plugin's name
   2. ./plugin options will output a list with plugin's options for goss to call. Like a ethtool plugin should output speed, is-bond, is-virtual etc
   3. ./plugin test-method test-target will output the test result. E.g: ethtool plugin, ./ethotool_plugin.sh is-virtual etho0 will output true
2. the plugin pass should be passed via cli args or config file, like: ./goss --plugins /usr/local/bin/ethotool_plugins then when goss start, it will create struct via ./plugin name and ./plugin options output.

Describe alternatives you've considered

For the implement of plugins, I have an idea. I use python to explain the code logic. We can talk about the implement and then I can create a new PR for this.

plugin:
  /tmp/test.sh:
      eth0:
        is-virtual: false
        is-bond: false

import yaml
import pprint
import json
import get_command_output
with open("test.yml") as f:
        data = yaml.load(f)

def get_options(executable_file):
        return get_command_output("%s options" % executable_file).strip().split(",")

def get_output_by_option(executable_file, option, target):
        return get_command_output("%s %s %s" % (executable_file, option, target)).strip()

for executable_file in  data["plugin"]:
        for execute_target in data["plugin"][executable_file]:
                for k,v in data["plugin"][executable_file][execute_target].items():
                        print "k=%s, v=%s, result=%s" % (k,v,json.loads(get_output_by_option(executable_file, k, execute_target)))
                        print(get_output_by_option(executable_file, k, execute_target) == v)

check_virtual() {
        echo "false"
}

check_bond() {
        echo "false"
}

case $1 in
        is-virtual)
                check_virtual $2
                exit 0
                ;;
        is-bond)
                check_bond $3
                exit 0
                ;;
        options)
                echo "is-virtual, is-bond"
                exit 0
                ;;
        name)
                echo "ethtool-plugin"
                exit 0
                ;;
        *)
                echo "Usage: $0 {options|is-virtual|is-bond}"
                exit 0
                ;;
esac