Si4713 external antenna connection

From http://falconchristmas.com/forum/index.php/topic,8996.msg100383.html#msg100383
Scuba provided a great walkthrough of adding an external antenna to the Si4713 board. I want to include this on the config page.

Ported from https://github.com/ShadowLight8/Si4713_FM_RDS/issues/10