Add IFTTT notification service

Add the ability to trigger a custom event on the Maker channel provided
by IFTTT service. With this trigger one can program actions on another
channels. Examples:

-Log backup notification to a google spreadsheet;
-Receive notification on devices with IFTTT application installed;
-Send e-mails
-Log to evernote
-Turn a Philips Hue light green for good backup or red on errors

IFTTT Maker channel accepts three values (named value1, value2, value3)
as part of the trigger. This implementation sends the content of
NOTIFICATION_CONTENT as "value1" parameter and let "value2" be defined
at
configuration file. At this time "value3" is not used.

Signed-off-by: Josenivaldo Benito Jr <jrbenito@benito.qsl.br>

duplicity-backup.conf.example

@@ -408,7 +408,7 @@ MAIL="mailx"     # default command for Linux mail
 # Third-party notification services. If NOTIFICATION_SERVICE is not provided, no
 # notifications will be sent.
 
-# Possible values for NOTIFICATION_SERVICE are: slack, pushover
+# Possible values for NOTIFICATION_SERVICE are: slack, pushover, ifttt
 NOTIFICATION_SERVICE=""
 NOTIFICATION_FAILURE_ONLY="yes" # send notifications only if there was an error while creating backup
 
@@ -422,6 +422,11 @@ SLACK_EMOJI="package"
 PUSHOVER_TOKEN="" # App token generated at pushover.net
 PUSHOVER_USER="" # User key from pushover.net
 
+# Provider: IFTTT
+IFTTT_KEY="" # Key for MAKER channel at IFTTT
+IFTTT_MAKER_EVENT="duplicity" # name the event to trigger at IFTTT Maker Channel
+IFTTT_HOOK_URL="https://maker.ifttt.com/trigger/$IFTTT_MAKER_EVENT/with/key/$IFTTT_KEY" # ONLY change this if IFTTT changes it
+IFTTT_VALUE2="" # general purpose value to pass to your maker channel (optional)
 
 # ------------------------------------------------------------------------------
 # TROUBLESHOOTING