Support asynchronous (promised-based) custom matchers

[bradvogel]

For our use case, we'd like to implement custom expressions that make database calls. An example of what we'd like to do (that doesn't work today):

sift.use({
  $existsInDb: async function(expectToExist, name) {
    const existsInDb = !!(await db.users.findOne({ name }));
    return existsInDb === expectToExist;
  }
});

const result = await sift({ $existsInDb: true }, ['bob', 'ben', 'brad']);

[skeggse]

FYI: we forked sift.js and implemented async support in mixmaxhq#1.

[crcn]

Just starting on this functionality in #157. The API will likely be:

const asyncFilter = sift({ $existsInDb: true }, {
  operations: {
    async $existsInDb(expectToExist, name) {
      const existsInDb = Boolean(await db.users.findOne({ name }));
      return existsInDb === expectToExist;
    }
  }
});

const results = await Promise.all(['bob', 'ben', 'brad'].filter(asyncFilter));