add 'sleep-extractor' option (closes #964)

(would have been nice if this were possible without code duplication)

docs/configuration.rst

@@ -208,6 +208,16 @@ Description Number of seconds to sleep before each download.
 =========== =====
 
 
+extractor.*.sleep-extractor
+---------------------------
+=========== =====
+Type        ``float``
+Default     ``0``
+Description Number of seconds to sleep before handling an input URL,
+            i.e. before starting a new extractor.
+=========== =====
+
+
 extractor.*.username & .password
 --------------------------------
 =========== =====

gallery_dl/job.py

@@ -59,6 +59,9 @@ class Job():
 
     def run(self):
         """Execute or run the job"""
+        sleep = self.extractor.config("sleep-extractor")
+        if sleep:
+            time.sleep(sleep)
         try:
             log = self.extractor.log
             for msg in self.extractor:
@@ -586,6 +589,10 @@ class DataJob(Job):
         self.filter = (lambda x: x) if private else util.filter_dict
 
     def run(self):
+        sleep = self.extractor.config("sleep-extractor")
+        if sleep:
+            time.sleep(sleep)
+
         # collect data
         try:
             for msg in self.extractor: