Add support for if_modified config setting

src/zasd.py

@@ -199,7 +199,7 @@ async def snapshot_creation_task(schedule, fs):
     serial = make_snapshot_serial()
     recursive = schedule['recursive']
 
-    if get_fs_flag(fs, tag):
+    if not schedule['if_modified'] or get_fs_flag(fs, tag):
         # Clear tag-modified flags for this tag on filesystem
         clear_fs_flag(fs, tag)
 
@@ -302,7 +302,6 @@ def load_snapshot_schedules():
 
 async def main_task():
     global config, event_loop, scheduler, fs_listeners
-
     
     scheduler = AsyncIOPriorityScheduler(
             event_loop = event_loop,