Support specifying browsers in the local.browsers api

Adding an optional function parameter to specify which browsers to return would fix the following issue (causing slow test ultimately in web-component-tester):
https://github.com/Polymer/wct-local/issues/21

A comment in the issue explains more clearly the cause of the issue:
https://github.com/Polymer/wct-local/issues/21#issuecomment-174602321

The parameter should be added here:
https://github.com/ekryski/launchpad/blob/master/lib/local/index.js#L59

This is just a quick proposal, I'm open to alternative solutions :).

Start at lib/local/index.js around line 59 and read the local.browsers API, then review the linked wct-local issue and comment for the slow-test behavior. Done means the API accepts an optional browser selection and returns only the requested browsers without changing existing calls; check the repository’s available tests for verification.

Written by the indexing model from the issue text.