log.GetLogger: add tag formating support

Signed-off-by: Alejandro Mery <amery@geeks.cl>
log: factor stderr stuff out into a backend module

Signed-off-by: Alejandro Mery <amery@geeks.cl>
log: rename LoggerMap to Group

Signed-off-by: Alejandro Mery <amery@geeks.cl>
log.Group: extend .Get() to support formatting

Signed-off-by: Alejandro Mery <amery@geeks.cl>