Add Text and Lens imports in Ex11-Ex14

[bgluzman]

From Exercise 11 onwards, I had to add this import to every file:

import qualified Data.Text as Text

And in Exercise 12, I had to add this import:

import Control.Lens

Moreover, every solution for those exercise has these imports already. I think adding these imports to the exercise skeletons would make solving them a bit more seamless.

Also, thanks for the great tutorial! It was really fun to work through.