Add jArchi support for CheatSheets

jArchi makes it possible to automate a lot of things. I think that, together with Cheat Sheets (which are an interesting but motly hidden feature of Archi) this would allow people to create really amazing things.

Thanks to @Phillipus's help, I've started to work on jArchi support for Cheat Sheets, and this has been much simpler than expected. So I've just submited some code in a new `cheatsheets` branch.

This code provides a very simple `RunArchiScriptCheatSheetAction` action which will run the script passed as an argument. The argument must contains the script path **relative to `__SCRIPTS_DIR__`**.

A basic _demo_ cheat sheet has been included. It expects the user to have created a `demo.ajs` script beforehand in the root scripts folder.

I'm planning to update this code so that the action takes another parameter to define the "scope" of the script, so that it becomes possible to run a script:
- through an absolute path
- included in another plugin (so this might adresses #82)
- by providing the actual code inside the cheatsheet itself

So opening this issue to discuss this feature.

Review the existing cheatsheets branch and the RunArchiScriptCheatSheetAction described in the issue, including its use of paths relative to __SCRIPTS_DIR__. Run the basic demo with a demo.ajs script in the root scripts folder, then define and verify the supported script scopes discussed in the issue.

Written by the indexing model from the issue text.