Add option to allow Custom Tags to control multiple property

Problem/Motivation

In some cases it is desirable to have a custom tag NOT split by the separator. Being able to control the "multiple" property setting on a custom tag would be useful.

Steps to reproduce

• Create any custom tag and set a "body" style field as it's token.
• Create a node and set text that includes commas into the field.
• The tag output will split the field value up on the commas creating multiple tags with chunks of the field value in them.

Proposed resolution

Add a new control on the Create/Edit custom tag form to control the "multiple" property.
Multiple? => TRUE, FALSE

Remaining tasks

I assume adjust the MetaTagCustomTag Plugin use that new configuration value in the same manner that it uses the htmlNameAttribute, htmlElement and htmlValueAttribute settings.

User interface changes

Create/Update custom tag form new select option.

API changes

Data model changes