also run network tests if EXTENDED_TESTING is set
split out "plugins to check" into its own attribute
remove _modules_before_build attr, pulling its functionality out into the callers and new _modules_prereq attr
split _check_modules into two: _prompt_if_stale and stale_modules
filter skips out of "modules" closer to source
New command: "dzil stale" (no options yet)
these plugins can be updated again
remove unused imports
Dist-Zilla-Plugin-PromptIfStale-0.016

          - new "dzil stale" command