Support custom async filters

[rogeliog]

This would be the second iteration of #15, once we generalize the typeahead code enough we can support custom async filters.

"watchPlugins": [
    ["jest-watch-typeahead/filename", {
        "key": "k",
        "prompt": "filter by files containing @tag",
        "filter": "<rootDir>/myFilter.js"
    }]
],

This would mean that the consumer can inject their business logic for dealing with the typeahead and we provide a great typeahead infrastructure that handles debouncing, loading indicator, etc.