[FEATURE] Gif previews for examples

I noticed that I frequently launch random examples just to see whether there is something that might fit my task. I guess I'm not the only one. Just looking at all(most?) examples on one page in the form of a list of gifs would seem like a huge timesaver.

**Describe the solution you'd like**
1. Generate gifs for all existing examples(I use asciinema) and save them somewhere in the repository

2. Update cursive/examples/Readme.md to display those gifs as embedded images next to each example name

**Pending questions**

1) where do you think such gifs should be present? /cursive/examples/assets/ ? /cursive/examples/previews/ ?

2) Any gif generating [agg config](https://github.com/asciinema/agg) that might be necessary for the final version?
For the attached gifs I used the following config
```
agg --theme solarized-dark --idle-time-limit 1 --fps-cap 3 "$1.cast" "$1.gif"
```
Reduced fps-cap just to keep the size of those files smaller.

3) Does the dimensions of those attached gifs look fine?
The idea was to make it smaller and accessible from smaller devices.