[todos] Add days to warn/error configuration in lint todo config

Currently there’s no serialized source of truth for configurations for days to warn/error. When creating todos, those days are configured in an ad hoc manner. We should extend our current configuration to include specific configurations for days to warn/error, per rule.

Example:

{
  "lintTodo": {
    "decayDays": {
      "warn": 5,
      "error": 10
    },
    "decayDaysByRule": {
      "link-href-attributes": { warn: 30, error: 90 },
      ...
    } 
  }
}

[scalvert]

Proposed changes:

1. Support both configuration styles:

   • package.json configuration (supports simple config where you only supply defaultDecayDays
   • .lint-todorc.js configuration (supports larger, more complicated configuration)

2. Expand configuration as per original suggestion:

   {
     "lintTodo": {
       "decayDays": { "warn": 5, "error": 10 },
       "decayDaysByRule" : {
         "link-href-attributes": { warn: 30, error: 90 },
         ...
       }
     }
   }
   

[rwjblue]

We originally chose to use package.json because the same settings would be read by multiple tools (not just ember-template-lint). I do agree that we might want to have a separate config file, but it can’t (directly) be .template-lintrc.js.