Create custom graph definitions using plugin meta

- Each plugin is invoked on agent startup with `MACKEREL_AGENT_PLUGIN_META` envirnoment variable set to 1.
- A plugin may print its meta information to stdout when the env is set, which starts with `# mackerel-agent-plugin` line followed by a TOML
- This meta information can be used to generate a custom graph definition

An example plugin:

```ruby
if ENV['MACKEREL_AGENT_PLUGIN_META']
  puts <<-META
# mackerel-agent-plugin
[graphs.dice]
prefix = "dice"
label = "My Dice"
[graphs.dice.metrics.d6]
label = "Dice(d6)"
[graphs.dice.metrics.d20]
label = "Dice(d20)"
  META
  exit 0
end

puts [ 'dice.d6',  rand( 6)+1, Time.now.to_i ].join("\t")
puts [ 'dice.d20', rand(20)+1, Time.now.to_i ].join("\t")
```
`Prefix` may not be used anywhere.