Quoteless route param in linkTo performs lookup

See: http://discuss.emberjs.com/t/upcoming-linkto-changes-ideas/1635

New behavior is wrapped in ENV.HELPER_PARAM_LOOKUPS flag,
named that way since the behavior will likely be applied to
actions as well.

The routeName param is bound. Saving bound contexts for a later PR
for now (potentially complicated).
Merge pull request #2905 from machty/string-link-to

Quoteless route param in linkTo performs lookup