Store mod slugs

[OgGhostJelly]

Stores mod slugs when they are added. Preliminary change for the gorilla-devs/ferium#462 PR that will fix gorilla-devs/ferium#439.

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have yours on file. In order for us to review and merge your code, please send a signing request to cla-requests@gdlauncher.com and add your github handle to contributors list.

[OgGhostJelly]

We require contributors to sign our Contributor License Agreement, and we don't have yours on file. In order for us to review and merge your code, please send a signing request to cla-requests@gdlauncher.com and add your github handle to contributors list.

I made a PR to add myself to the contributers list (gorilla-devs/clabot-config#13) and emailed a signing request to cla-requests@gdlauncher.com, I guess I'll be waiting for a response? I haven't ever signed a CLA before so I'm not really sure how this is supposed to go.

[theRookieCoder]

I don't think the CLA signing is working right now, but anyways I'll merge it now. We'll deal with it later.