Add custom prefix

[Theosakamg]

Can custom prefix on tdm.cfg :

• prefix_cmnd=xxx // Tasmota-MQTT-Prefix1 (default = cmnd)
• prefix_stat=yyy // Tasmota-MQTT-Prefix2 (default = stat)
• prefix_tele=zzz // Tasmota-MQTT-Prefix3 (default = tele)

[Theosakamg]

Fix #86

[jziolkowski]

Closing because I'm working on a solution based on per-device settings for custom prefixes. Thanks anyway.