add fullpath option for write file's path

[Allenhe123]

when using log.c to dump information, the __FILE__will be very long if the .c file's path is deep. So i add a option to enable/disable writing the file's full path.

[Balth-D]

Hello, I found your pull-request useful (thread example) and merged it to my fork ! Feel free to propose changes, thanks for your contribution :smiley: https://github.com/BDeliers/log.c