Refactor markdown

>in a new branch, refactor markdown rendering to move our logic into the post.rb file. You should render markdown like so:
```
<%= post.markdown_title %>
```
>Define a Post#markdown_title and Post#markdown_body method in app/models/post.rb, like so:
```
app/models/post.rb
def markdown_title
  render_as_markdown title
end
```
```
def markdown_body
  render_as_markdown body
end
```
>The actual functionality of rendering as markdown should be defined in a private Post#render_as_markdown method, to keep the above two methods DRY. This should allow you to get rid of the markdown helper method.