Fix use of {{partial}} inside block helper like {{each}}

Before, `{{partial}}` was incorrectly looking for the `_app` property;
it could find it in the helper context (`this._app`), but not in the
helper options (`options.data._app`), which are utilized by block
helpers like `{{#each}}` for passing through the app context.

This adds a `getProperty(key, context, options)` method which DRYs up
that logic.

Also cleans up the context logic in `{{partial}}`.
Merge pull request #4 from airbnb/fix-partial-in-each

Fix use of {{partial}} inside block helper like {{each}}