Add "feature request" issue template

[w-k-jones]

It would be useful to have an issue template for requesting new features/functionality in tobac along with the templates we currently have. Some potential cheeckboxes/questions:

• [ ] Is this feature within the scope of tobac?
• [ ] Is this feature part of the tobac development goals? (link to roadmap)
• Are you working on this feature yourself, or would you like assistance on this project?
• Will this feature require changes to the tobac API, and will it change existing workflows?
• How long do you expect it to take before this feature is ready for release?

[JuliaKukulies]

Good idea!