closes#4439
- adds basic get helper which works with the current API
- allows theme developers to make requests against the API
- supports block params and @error message
- includes 100% test coverage using posts
----
The `{{#get}}` helper is an asynchronous block helper which allows for making
requests for data from the API. This allows theme developers to customise the
data which can be shown on a particular page of a blog.
Requests can be made to the posts, tags or users API endpoints:
```
{{#get "posts" limit="3"}}
{{#foreach posts}}
<a href="{{url}}">{{title}}</a>
{{/foreach}}
{{/get}}
```
The `{{#get}}` helper must be used as a block helper, it supports `{{else}}`
logic, for when no data matching the request is available or if an error has
occurred:
```
{{#get "posts" tag="photo"}}
...
{{else}}
{{#if @error}}
<p>Something went wrong: {{@error}}</p>
{{else}}
<p>No posts found</p>
{{/if}}
{{/get}}
```
The helper also supports block params, meaning the data it outputs can be
given a different name:
```
{{#get "posts" featured="true" as |featured|}}
{{#foreach featured}}
...
{{/foreach}}
{{/get}}
```
Please Note: At present asynchronous helpers cannot be nested.
