Add ability to request multiple channels from /kraken/channels

[SReject]

It’d be nice if we could request information pertaining to multiple channels via a single HTTP request; similar to how /kraken/streams work.

Calls against /kraken/channels?channel=test_channel1,test_channel2 would return an array of channel objects akin to:

{
    "channels": [
        {
            // ...
            "name": "test_channel1",
            // ...

            "links": {
                // ...
            }
        }, {
            // ...
            "name": "test_channel2",
            // ...

            "links": {
                // ...
            }
        }
    ],
    "links": {
        // ...
    }
}