Please add a warning and cancel option when options change

If I accidentally run "stack install" with different options (add/remove -profile, --extra-lib-dirs, etc), everything is reset and rebuilt.... For our project, this can take a long time, 10-20 minutes. I understand why you have to do this, and agree that it is the correct decision.

However, a warning that would let us cancel would be great.... Especially since I often just mistype or forget a particular option (like -profile), and what should have been a 30 second compile suddenly becomes a 20 minute ordeal. I do this a lot....

Something like this would be great:

```
> stack install
Warning: You have changed build options, and stack is about to delete all prebuilt
packages and rebuild from scratch (this can take a long time). Do you wish to continue? (Y/n)
```