Support specifying browsers in the local.browsers api

[panuhorsmalahti]

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 :).

[daffl]

This should already be fixable by specifying the browsers, as added in #52 (also see #51). Additionally, the test could just directly specify what browsers it wants to run (e.g. launch.firefox etc.) without calling .broswers (which in Windows triggers a lengthy search for all browsers).

[panuhorsmalahti]

Yeah, LAUNCHPAD_BROWSERS is a working workaround, but it's an extra unnecessary step for wct users, since the wct configuration file already has a property for specifying the browsers:
https://github.com/Polymer/web-component-tester/blob/master/runner/config.js#L76

[daffl]

I would recommend changing the wct tests to use that configuration in detect then. local.browsers only needs to be called if you want a list of all locally installed browsers. If they are already specified it doesn't need to run at all, you just call launch.firefox, launch.chrome etc. directly. It will error if a user doesn't have the browser installed but I would assume that having the browsers specified in the configuration are a requirement to run the tests.

[panuhorsmalahti]

That's a good idea. detect should only use local.browsers if the user didn't specify which browsers to use.