Enable compressed connections to the IMAP server

Added the configuration setting usecompression for the IMAP repositories.
When enabled, the data from and to the IMAP server is compressed.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

Changelog.rst

@@ -9,6 +9,7 @@ OfflineIMAP v6.5.6 (YYYY-MM-DD)
 ===============================
 
 * Add knob to invoke folderfilter dynamically on each sync (GitHub#73)
+* Add knob to apply compression to IMAP connections (Abdó Roig-Maranges)
 
 
 OfflineIMAP v6.5.5 (2013-10-07)

offlineimap.conf

@@ -479,6 +479,11 @@ remoteuser = username
 # idlefolders = ['INBOX', 'INBOX.Alerts']
 #
 
+# OfflineIMAP can use a compressed connection to the IMAP server.
+# This can result in faster downloads for some cases.
+#
+#usecompression = yes
+
 # OfflineIMAP can use multiple connections to the server in order
 # to perform multiple synchronization actions simultaneously.
 # This may place a higher burden on the server.  In most cases,

offlineimap/imapserver.py

@@ -422,6 +422,10 @@ class IMAPServer:
                         self.passworderror = str(e)
                         raise
 
+            # Enable compression
+            if self.repos.getconfboolean('usecompression', 0):
+                imapobj.enable_compression()
+
             # update capabilities after login, e.g. gmail serves different ones
             typ, dat = imapobj.capability()
             if dat != [None]: