Added themes.json and some extras

[RunDevelopment]

In preparation for prism#2175, I added a way to get 1) all themes and 2) the name of each theme. So I added a themes.json with that information.

From here on it escalated a little.

Since themes.json and README.md both contained that same information, I added the author info to themes.json and made it so that gulp now generates the list of available themes from themes.json.

With those changes, I updated the "Added a new theme" section in README.md and the section got quite long, so I moved it into CONTRIBUTING.md.

And since I already added a file specifically for information for contributors, I might as well include a few guidelines and requirements we have for themes to guarantee that they work with Prism and its plugins.

But now we have rules with no way to enforce them! So I added a new check to gulpfile.js which using JSDom verifies that all themes fulfill all requirements.
And guess what! One existing theme didn't fulfill a requirement, so I fixed it.

That's how we got here.

---

Features:

• New themes.json which contains basic information about all themes.
• The "Available Themes" section is automatically generated.
• New and expanded contributing guide.
• New checks for themes to ensure compatibility with Prism and its plugins.