When a mod adds a dependency to its manifest (see #285), let it specify whether it's required (default true):

{
  ...,
  "Dependencies": [
    {
      "UniqueId": "EntoaroxFramework",
      "IsRequired": false
    }
  ]
}

If the dependency is available, it should be loaded before this mod; otherwise the dependency should be ignored.

Split from #248.

Done in develop for the upcoming SMAPI 2.0 release.