synchroniser : add arbitrary synchronisation arguments

Add the option to give arbitrary arguments to the unison call.
These arguments must be passed as a list to sync and will be given to
unison as is.
This is a prerequisite for using the backup system of unison as the
arguments for backup will only be given when synchronising the files and
not the links.

src/unisync/synchroniser.py

@@ -150,7 +150,9 @@ class Synchroniser:
                 )
 
     def sync(self, remote_root:str, local_root:str,
-             paths:list=[], ignore:list=[], force:bool=False) -> int:
+             paths:list=[], ignore:list=[], force:bool=False,
+             other:list=[]
+             ) -> int:
         """Performs the synchronisation by calling unison.
 
         Args:
@@ -162,6 +164,12 @@ class Synchroniser:
                 If you need to ignore some specific files use the arguments.
             force: Force all changes from remote to local.
                 Used mostly when replacing a link by the file.
+            other:
+                Other arguments to add to unison.
+                These arguments will only be used for this sync which is not
+                the case for the ones in self.args_bool and self.args_value.
+                They will be added to the command as is no - in front.
+                For exemple backups are implemented using this argument.
 
         Returns:
             the unison return code see section 6.11 of the documentation
@@ -192,6 +200,9 @@ class Synchroniser:
             command.append(remote_root)
             command.append("-batch")
 
+        for arg in other:
+            command.append(arg)
+
         proc = subprocess.Popen(command)
         ret_code = proc.wait()
         return ret_code