Honor .rsync-filter files

[TylerRick]

Auto-add --filter='dir-merge /.rsync-filter' (which is also available with -F) so users don't have to go out of their way to manually add that option:

       -F     The -F option is a shorthand for adding two --filter rules to your command.  The first time it is used is a shorthand for this rule:

                  --filter='dir-merge /.rsync-filter'

              This tells rsync to look for per-directory .rsync-filter files that have been sprinkled through the hierarchy and use their rules to filter the files in the transfer. 

This in turn makes it easier for users to exclude problematic files (#71) simply by adding a file with - problem_file.

See also #64 and #84 for where this would be useful.

[MrAMS]

Helpful idea 😃