Added parameter to pageaddvar to disabled any internal processing of content by plugin

This PR adds a parameter to allow for disabling of internal processing by the pageaddvarplugin. Under twig we can do the processing with a proper parser but this is suitable fix for the legacy view code.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #643
| Refs tickets  | 
| License       | MIT
| Doc PR        |