Make {{intlGet}} a generic helper to get anything on options.data.intl
Add support for `format()` methods passed to {{intlMessage}}

This adds support for an IntlMessageFormat instance's `format()` to be passed to the {{intlMessage}} helper by reference. Since the `format()`'s `this` is pre-bound to the instance, this is a valid use.
Only export `registerWith` which allows helpers to reference Handlebars

This change reduces this modules exports to only the `registerWith()`
function to make sure when the helper functions are defined, they're
within a closure where they have access to `Handlebars`.

This allow for the use of Handlebars' utilities like `SafeString`, `escapeExpression()`, etc.
Fix intl-messageformat GitHub dependency in package.json
Add {{intlHTMLMessage}} helper

This is like {{intlMessage}} but allows for the message pattern to contain HTML. When using this helper, all of the `options.hash` values will be HTML-escaped before they are passes to the message format's `format()` method, and the result will be declared "safe" via `Handlebars.SafeString`.