Multiple template support

[viroge]

Implements #27

Added multiple template support:

• Changed zabbix_template custom field type to Multiple selection
• Updated the script and readme file

[TheNetworkGuy]

Although i do appreciate your contribution this is not the direction that i want to take with this feature.

If we go with this method then each time the user will need to create a new template in Zabbix, after that add it to the choices field within Netbox and finally set it under the device type. Those are 3 steps while our current method is only 2. I'm working on an implementation to list the templates under config context which would create flexibility for the end user way better.