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

configs/samples/logger.conf.sample

@@ -109,6 +109,10 @@
 ; levels, and is enclosed in square brackets. Valid formatters are:
 ;   - [default] - The default formatter, this outputs log messages using a
 ;                 human readable format.
+;   - [plain]   - The plain formatter, this outputs log messages using a
+;                 human readable format with the addition of function name
+;                 and line number. No color escape codes are ever printed
+                  nor are verbose messages treated specially.
 ;   - [json]    - Log the output in JSON. Note that JSON formatted log entries,
 ;                 if specified for a logger type of 'console', will be formatted
 ;                 per the 'default' formatter for log messages of type VERBOSE.