Add Fate die support

[xudonax]

Summary

This is kinda rough, but should work fine. A physical Fate die is a d6 containing two - signs, two + signs, and two blank sides. This is emulated using -1 for the - sign, +1 for the + sign, and a 0 for the blank side. The result is that we can still add them up (so 10dF work), subtract them, keep highest etcetera.

Also added a FateLiteral class that's optional, it's just there to make the result (when looking at result. Expr) a bit nicer.

Resolves #8

Checklist

PR Type

• [x] This PR is a code change that implements a feature request.
• [ ] This PR fixes an issue.
• [ ] This PR adds a new feature that is not an open feature request.
• [ ] This PR is not a code change (e.g. documentation, README, ...)

  Other

• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] If code changes were made then they have been tested.
• [x] I have updated the documentation to reflect the changes.

[xudonax]

I couldn't find where the readthedocs site is generated from, but I did update the README, I hope that's enough to tick the "I have updated the documentation to reflect the changes"