feat(custom): custom render prismic type

[mrloop]

See updated README and tests for examples.

Custom rendering of prismic type using custom component for prisimc type.

Pass a custom component name to be used to render a prismic type. For example to custom render the 'group-list-item' and 'hyperlink' types

<Prismic::Dom                                                                             
  @group-list-item='my-list'                                                              
  @hyperlink='my-hyperlink'                                                               
  @nodes={{@myPrismicDoc.data.myRichText}}                                                
/>                                                                                        

my-list.hbs

<h1>My List</h2>                                                                          
<ul>{{yield}}<ul>                                                                         

my-hyperlink.hbs

<a href={{@node.element.data.url}}>{{yield}}</a>                                          

Useful Links

• https://prismic.io/docs/technologies/html-serializer-javascript#adding-the-html-serializer-function

Alternatives

Explored using named blocks https://github.com/qonto/ember-prismic-dom/compare/em-custom, it would give a nicer interface, allowing you to declare custom type rendering in the same template as Prismic::Dom is used. However the implementation was going to be very verbose and broken. https://github.com/qonto/ember-prismic-dom/compare/em-custom#diff-0b7a759bb6f26f39ae19fb331cc235b7c6588d834f0204c37e2f350306fa6d33R2 errors Syntax Error: {{yield}} can only accept a string literal for the to argument (at ember-prismic-dom/components/prismic/children.hbs on line 2 column 20)

Closes #3