Add `data` property to `FormState`

[mcuppi]

trafficstars

This commit adds a data property to the FormState object. The property serves the same purpose as the data property in the FieldState object: It is a place for arbitrary values to be placed by mutators.

[mcuppi]

@erikras I believe I've followed all of the contributing guidelines. If I missed any, please let me know. Additionally, I could write tests for this if you want.

[gertdreyer]

@mcuppi Could you add tests and an example, please?

[mcuppi]

@gertdreyer I apologize of the long silence. I'll try to add tests soon.