Configurable preferred codec

[Suxsem]

Hi,
currently, the first codec is chosen as preferred codec:

pyVoIP/pyVoIP/RTP.py

Lines 313 to 318 in dd2c83c

	Select the first available actual codec to encode with.
	TODO: will need to change if video codecs
	are ever implemented.
	"""
	self.preference = assoc[m]
	break

Compatible codecs list is hardcoded in init.py, so preferred codec will always be PCMU

pyVoIP/pyVoIP/__init__.py

Line 100 in dd2c83c

RTPCompatibleCodecs = [PayloadType.PCMU, PayloadType.PCMA, PayloadType.EVENT]

I need a way to use PCMA as preferred codec.

@tayler6000 I'm going to prepare a PR to add this feature, do you have any suggestion in how to expose this configuration? Thank you