New function findKeys proposition

I'd like to use a function but it seems it's missing. I didn't find any issue for this, and I didn't find it in https://github.com/documentcloud/underscore-contrib (but I'm not sure how to search efficiently here)

function description:
### findKeys
`_.findKeys(object, predicate, [context])`
Returns an array of keys where the predicate truth test passes (or empty array).

`_.findKeys({ a: 1, b: 2, c: 3, d: 2}, function (val, key) {
return val === 2;
})`
` => ["b", "d"]`

`_.findKeys({ a: 1, b: 2, c: 3, d: 2}, function (val, key) {
return val === 4;
})`
` => []`

Am I the only one who would need this ? If not, i can do a PR

No file or test is named in the issue. Start by locating the object-iteration implementation and existing tests, then use the proposed _.findKeys signature and examples as the behavioral specification; done means matching keys are returned in an array and no matches return an empty array.

Written by the indexing model from the issue text.