Write usage example using Literate Haskell

[chshersh]

We would like to have tutorial-like examples in Iris, showcasing project features.

The proposed structure of the example:

1. Rename app directory to examples
2. Create a separate directory with an example name for each separate example.
3. Each example also should have a corresponding executable stanza in the iris.cabal file.
4. Current example should have name playground
5. Each example (except playground) should be written using markdown-unlit. The structure of an example:
   • README.md with the example content
   • Main.lhs as a symbolic link to the README

Possible examples:

• [x] playground
• [x] Simple grep: search lines containing substring in a given file, print coloured file name to stderr, lines numbers to stderr and with lines to stdout (to mimic the output of ripgrep)

This should be enough for the start but later we can add more examples 🙂

[Dponya]

I'll work on this

[chshersh]

@Dponya Go for it! 👏🏻