Custom multipart field types

Included issues:

• guardrail-dev/guardrail#70 Custom types are not supported in multipart forms
• guardrail-dev/guardrail#71 Show instances for Float and Double are missing
• guardrail-dev/guardrail#72 Multipart terms result in a compilation failure in http4s

Contributors:

• @blast-hardcheese
• @sullis