logger.c: Added a new log formatter called "plain"

Added a new log formatter called "plain" that always prints
file, function and line number if available (even for verbose
messages) and never prints color control characters.  It also
doesn't apply any special formatting for verbose messages.
Most suitable for file output but can be used for other channels
as well.

You use it in logger.conf like so:
debug => [plain]debug
console => [plain]error,warning,debug,notice,pjsip_history
messages => [plain]warning,error,verbose

Change-Id: I4fdfe4089f66ce2f9cb29f3005522090dbb5243d