Allow to use only specific operators

It may be useful to expose all operators separately and allow users to create own instance of sift that supports only specific operators.

benefits form this:

1. We can reduce bundle size on client (by using tree-shaking feature)
2. We can restrict users to use only allowed operators (e.g., forbid usage of $where)
3. Potentially separation on operators and execution engine may allow to implement processing of async conditions easier

[stalniy]

Exactly!

[crcn]

Do you mean to import operators separately into sift? E.g:

import sift from 'sift';
import * as operators from 'sift/operators';

const filter = sift({ operators });

Additionally, operators could be separated out into individual files:

import sift from 'sift';
import $where from 'sift/operators/$where';
import $in from 'sift/operators/$in';
import $eq from 'sift/operators/$in';

const filter = sift({ operators: {$where, $in, $eq} });