This adds an optional parameter to find_rf_packet(), along with a
corresponding --rflearn parameter (defaulting to 433.92) to
broadlink_cli that specifies the frequency to tune to, rather than
requiring the frequency be found via sweeping. This is almost mandatory
for certain types of remotes that do not repeat their signals while the
button is held, and saves significant time when the frequency is known
in advance or when many buttons are to be captured in a row.

Additionally:

- A get_frequency() API is added to return the current frequency the
  device is tuned to.

- A check_frequency_ex() API is added to perform functions of both
  check_frequency() and get_frequency() in a single call.

- broadlink_cli --rfscanlearn will now report the current frequency at 1
  second intervals during sweeping, and will report the frequency it
  finally locks on to.