Regularise the config for plugins

[alanz]

At the moment the Config type is

data Config =
  Config
    { hlintOn             :: Bool
    , maxNumberOfProblems :: Int
    } deriving (Show)

As we add more diagnostic providers, we can add a specific config for each.

Or, we can make the config something like

data Config =
  Config
    { diagnostics :: Map PluginId  Bool
    , maxNumberOfProblems :: Int
    } deriving (Show)

This means that nothing has to change when we add a new plugin that does diagnostics, and we can filter out the disabled ones in requestDiagnostics already.

This can potentially apply for code actions, hover, and symbols. And be extended as new items arise.

[lukel97]

I would like to also see an extensible point for custom plugin configs, so we can specify

"hlint": {
  "diagnostics": true,
  "maxNumber": 10
}

as a data type in Hlint.hs

data HlintConfig =
  HlintConfig
  { diagnostics :: Bool
  , maxNumber   :: Int
  }

This would help complete #772