Add support for selecting XB-200 filter [issue 288]

Added option to bladeRF-cli command line parsing to select XB-200 RX or TX filters:

xb filter 200 xx yy

where:
  xx is tx or rx
  yy is the filter to be selected (50, 144, 222, custom, auto_1db, auto_3db)

example:
bladeRF> xb enable 200
 Enabling transverter board
 Transverter board successfully enabled
bladeRF> xb filter 200 rx 144
 Transverter board rx filter successfully set to 144
bladeRF>