lyrics: Configuration option for ReST writing

[doronbehar]

The title says it all. I think it could be great to have a lyrics.default_directory entry in the configuration file for a default directory in which the lyrics file will be written, instead of always specifying it manually with -r or --write-rest. Usually I run this command not from a specific directory so this could be useful.

[sampsyo]

Good point! Perhaps the simplest way to address this would be to just add a corresponding config option with the same semantics as the -r command-line flag.