make exercise area wider or configurable

[thoraxe]

The narrow width of the area for exercises really makes certain things difficult -- especially displaying long shell commands or wide command output.

• can we make the exercise area wider?
• can we make the width of the exercise area configurable?

[thoraxe]

If we made it possible to load additional CSS files this could be pretty easy.

[marekjelen]

The area is now wider in the ruby branch and flexes based on the windows size.