Treat {{#each}} as {{#each this}}

Now the following is supported:

    {{#each}}Hello, {{name}}{{/each}}

This should alleviate the silliness that all of the following are
(often) the same:

    {{#each this}}...{{/each}}
    {{#each controller}}...{{/each}}
    {{#each content}}...{{/each}}
Merge pull request #2709 from machty/single-arg-each

Treat {{#each}} as {{#each this}}