Group all *.done.txt files into an optional subdirectory

[camilbancioiu]

This PR adds the possibility to write *.done.txt files to a local subdirectory (e.g. done). This way, all the done.txt files are grouped together in one place, which is helpful if you already have many *.todo.txt files.

This subdirectory is specified by g:todo_done_subdir, which can remain unspecified if the default behavior is desired. But if this option is specified, the subdirectory must already exist, manually created.