Stringifying linkTo args to prep for push for path lookups

We'll be making {{linkTo foo}} perform a lookup of the target
path on the present context for the 'foo' property, and we have
lot of references in the guides that advertise the incorrect
(albeit prettier) format of omitting the strings.
Merge pull request #559 from machty/stringify-link-to

Stringifying linkTo args to prep for push for path lookups