Fix handling partials from standalone < tags.

If the < tag is standalone, each non-empty line of the partial should be indented before it is rendered. The [spec](https://github.com/mustache/spec/blob/v1.1.2/specs/partials.yml#L12-L14) actually says all lines, not specifically non-empty ones. But the spec tests seems to expect only non-empty ones to be indented, and I've verified that other implementations (e.g. Pystache) do it this way.

3 more test cases now pass. The remaining failures are all related to dotted names and implicit iterators, which are not supported yet.  I have no pressing need for them so I might take a break here :)