Form example compatibility w/ 1Password extension

[craiglyons]

Problem:
The form example does not work with the 1Password browser extension in Chrome and Firefox. It may also be affected by other browsers / extensions.

Solution:
Wrap inputs in a span element. This is applied to the function that generates the input elements, which gives the fix a very small footprint.

Related to:
elm/virtual-dom#147
evancz/elm-architecture-tutorial#132

[evancz]

I think it's important to not have extra stuff, and independent of the importance of this topic, I think getting into compatibility with extensions is out of scope for this part of the book.