Allow specific CacheMap key type

[AdamHerrmann]

I would like to be able to use both the cacheKeyFn and cacheMap options in typescript. Currently the following fails to compile because CacheMap has the wrong key type:

interface Id {
  id: string;
}

const loader = new Dataloader<Id, Id>(async i => i, {
  cacheKeyFn: (k: Id) => k.id,
  cacheMap: new Map<string, Promise<Id>>(),
});

This change allows the following:

const loader = new Dataloader<Id, Id, string>(async i => i, {
  cacheKeyFn: (k: Id) => k.id,
  cacheMap: new Map<string, Promise<Id>>(),
});

I'm fairly certain this change is backward compatible, so this should continue to work:

const loader = new Dataloader<Id, Id>(async i => i);

Fixes #197

[coveralls]

Coverage remained the same at 100.0% when pulling 3cdad8a on AdamHerrmann:fix/types into 420e62f on facebook:master.

[leebyron]

Coverage failure is related to #210. This change dropped the total number of lines of code so coverage percentage went down