Optionally prefix name with a namespace string

[leff]

As we move to id attributes, we need to be able to ensure uniqueness across the whole page. On pages where the rendered markdown might be part of a larger document, we can't guarantee uniqueness through internal means alone. We can provide a way for users to ensure uniqueness by allowing them to prefix the generated name with a namespace string.

By default, the prefix will be an empty string.

Definition of Done:

1. A prefix option is supported
   • if the option is not set, no change is made to the raw name slug
   • if the option is set, it is separated from the raw slug by a dash. Ie prefix-rawslug
2. The feature is documented
   • It's use is encouraged, all examples have it set.