Scan RF code of specified frequency

[banboobee]

Note that this PR requires kiwi-cam/broadlinkjs-rm#23

This PR adds a function to scan RF code of specified frequency for learn-code accessory. Use new config keyword 'frequency' to specify the frequency as follows.

 {
                    "name": "...",
                    "type": "learn-code",
		    "frequency": 339.9,
		    "scanRF": true
}