"Cut" functionality

[xepozz]

We need cut functionality that will show the first part of the post and hide the second part.

This is necessary so that people open posts and delve into them on a separated page, not on the page with the list of posts.

I think for the first time we can use just <hr> tag as a delimiter.
It's easy to implement and won't affect original look (in github issue)

Example

First part

***

Second part

What we will see on the page with the list of posts

First part

What we will see on the post's page

First part

---

Second part