(PUP-1176) Add evaluator feature switch for future/current evaluator

This adds an --evaluator future/current setting to control if
the future evaluator should be used or not when using the future
parser. It is set to "on" (i.e. "future") by default. Setting it to
"current" turns off the future evaluator.

This commit only contains the setting itself.
(PUP-1176) Add feature switch for evaluator to ParserFactory

This adds the --evaluator future/current switch to the ParserFactory.
It is only in effect when using --parser future. By default the
future evaluator is used, but the older transforming evaluator can
be used by also stating --evaluator current.
(maint) Fix typo in documentation for --parser and --evaluator

A fature is not spelled that way.
Merge pull request #2209 from hlindberg/pup-1176_evaluator-feature-switch

(PUP-1176) Add evaluator feature switch