Add support for Pushbullet notifications

This commit adds the zed_notify_pushbullet() function and hooks
it into zed_notify(), thereby integrating it with the existing
"notify" ZEDLETs. This enables ZED to push notifications to your
desktop computer and/or mobile device(s). It is configured with the
ZED_PUSHBULLET_ACCESS_TOKEN and ZED_PUSHBULLET_CHANNEL_TAG variables
in zed.rc.

https://www.pushbullet.com/

The Makefile install-data-local target has been replaced with
install-data-hook. With the "-local" target, there is no particular
guarantee of execution order. But with the zed.rc now potentially
containing sensitive information (i.e., the Pushbullet access token),
the recommended permissions have changed to 0600. The "-hook" target
is always executed after the main rule's work is done; thus, the
chmod will always take place after the zed.rc file has been installed.

https://www.gnu.org/software/automake/manual/automake.html#Extending

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>