Include all argument names function

[AnataBakka]

Feature summary (what you would like to be able to do and where):
include = currently just includes the values. I would need a includekeys = {templateName} that gets me all paramater names of that template.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Currently, the only way to reliably get all argument names from a template is to use a surrogate template with lua's getParent().args. Problem with that is that it automatically expands each argument, which i don't want to, and dpl should be able to parse for the names without expanding the values better than i can.

Benefits (why should this be implemented?):
Better performance and higher usage.

[Universal-Omega]

Just a note as I prepare to release stable DPL4 in a day or two, I was going through issues. I will consider this, but for now it is unlikely to implemented in the near future but long term I will consider this.