Publicize style structs (formatFont, formatStyle and all internal structs separatly)

[therox]

I think it'll be very comfortable and will minimize errors not to generate style json by hands, but (at least) marshal it for default struct.

[xuri]

Thanks for your advice. I will make changes to assessment the APIs effect.

[mlh758]

I would also like to see this change. I had to dig into the source code to figure out what style properties to set in order to align font for example. If the struct was pubic it would have been a lot easier to figure out and it would also help prevent typos.

[trende-jp]

I also think this would be a big improvement. There is nothing to lose by making it available and, in fact, I think it will make discovering how to interact with this library much simpler.

[trende-jp]

#470

Pending pull request here.

[xuri]

I exported Style structs to allow create the style for cells by given JSON or structure. Ref: 9e2318c