Recognize configuration for idlefolders

Mark this option as experimental and document its shortcomings in
MANUAL.rst.

This code was originally by James Bunton <jamesbunton@fastmail.fm>.

Signed-off-by: Ethan Glasser-Camp <ethan@betacantrips.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/IMAP.py

@@ -79,9 +79,13 @@ class IMAPRepository(BaseRepository):
         self.imapserver.close()
 
     def getholdconnectionopen(self):
+        if self.getidlefolders():
+            return 1
         return self.getconfboolean("holdconnectionopen", 0)
 
     def getkeepalive(self):
+        if self.getidlefolders():
+            return 29*60
         return self.getconfint("keepalive", 0)
 
     def getsep(self):
@@ -163,8 +167,14 @@ class IMAPRepository(BaseRepository):
     def getreference(self):
         return self.getconf('reference', '""')
 
+    def getidlefolders(self):
+        localeval = self.localeval
+        return localeval.eval(self.getconf('idlefolders', '[]'))
+
     def getmaxconnections(self):
-        return self.getconfint('maxconnections', 1)
+        num1 = len(self.getidlefolders())
+        num2 = self.getconfint('maxconnections', 1)
+        return max(num1, num2)
 
     def getexpunge(self):
         return self.getconfboolean('expunge', 1)