Support selective DSL keywords on import?

[xsawyerx]

Currently you cannot pick which DSL keywords would be imported from Dancer2. This also confuses static analyzer Perl::Critic. I suggest we allow this.

We currently check for the following:

• :script. :syntax, :tests: No op.
• :nopragmas.
• appname, followed by a string, representing a class.
• dsl, following by a string representing a class.
• Anything prefixed with !, meaning "do not import this thing".

We can verify anything else is a keyword and if so, support it being used as a filter instead. This would be simpler and much more consistent with general importing conventions.