Support asynchronous (promised-based) custom matchers #138
Description
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']);