Allow default values for custom replacement properties in templates

[AdmiringWorm]

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.

[ferventcoder]

That's kind of interesting. I wonder how difficult it might be to rework this. I would see it maybe as =