Allow default values for custom replacement properties in templates

When we are creating our own custom package templates with custom replacement properties, I think it would be useful to be able to specify a default value to be inserted if a user haven't passed in the value. (A fallback value, if you will).

The way I am thinking this could be used by the one create the template is something like this:
```
[[extensionId::FALLBACK_VALUE]]
```
or something similiar to that.

No file, test, or entry point is identified in the issue. First locate the code that processes custom replacement properties in package templates and determine how replacement syntax is parsed. Done means a template can specify a fallback value that is used when the corresponding property is not supplied.

Written by the indexing model from the issue text.