Prompt the user on which linter to use from the Yeoman generator

[doug-wade]

Currently, the generator gives you xo as the linter no matter what, but this makes react-server seem bigger and scarier to new devs, and more inflexible to experienced devs. We should prompt the user for which linter, if any, they want to use (including eslint and maybe jshint or jslint if anyone is still using them, maybe closure, who knows what all linters we might want to support), and only use a linter if the user opts in.