Show confirm dialog before execute!!!

The addin does not give a warning before executing clojure code.
This is obviously dangerous, see for example:

```
=CLJEVAL("(use 'clojure.java.io) (with-open [wrtr (writer '''/tmp/test.txt''')]   (.write wrtr '''line to be written''')) ")
```

A confirm dialog should be shown (only once when opening the spreadsheet) before running any code.
