Made structs available publically and ran linter

[nokka]

• Added linting and updated the problems it highlighted
• Added header, attributes skills and items to the public interface.

[nokka]

@dschu012 I wanted to make some updates to the library since I wrote it in 2016 as my first go project! Like adding some of the structs to the public interface to ease the usage, running a linter and also add go.mod eventually, I want to avoid doing a v2 since I have to copy the entire lib into a v2 folder so people can use it as github.com/nokka/d2s/v2 but instead I was thinking about adding gomod for v1, but I'm not sure if this will cause any problems downstream for users like you since I altered the interface of the package even if I do a new tag such as v1.2.0. Do you have any concerns with this?