BlogRoll not using "count" property

## Feature request

**What is the expected behavior?**
The BlogRoll has a count property that doesn't look to be implemented.

**What is motivation or use case for adding/changing the behavior?**
We should be able to use this count property to determine the number of blog posts to return. For example, I have a "latest from the blog" section that only shows the latest 3 blog posts.

**How should this be implemented in your opinion?**
The BlogRoll component should receive props and pass that to the BlogRollTemplate. That count can be used to slice the initial data or as part of the .map() function.

I'm doing the latter with a ternary operator to keep the count property optional.
`{posts && posts.slice(0, (this.props.count != undefined ? this.props.count : this.props.data.length)).map(({ node: post }) => { ... }`

**Are you willing to work on this yourself?**
yes