feat: add per-call ContentType override via CallOptions

[caueasantos]

Summary

Adds optional per-call options to Data, HTML, JSON, JSONP, Text, and XML, letting callers override the response Content-Type for a single call without spinning up a second Render instance.

Closes #109.

Changes

• New CallOptions struct (with ContentType field) accepted as a trailing variadic argument by Data, JSON, JSONP, Text, and XML.
• HTMLOptions gains a ContentType field, propagated through prepareHTMLOptions.
• Internal resolveContentType helper centralizes the override logic.
• When an override is supplied, the configured charset is not appended — callers are expected to provide a fully-formed Content-Type value.
• All existing call sites continue to compile unchanged (variadic, backward compatible).

Testing

• go build ./...
• go vet ./...
• go test -race ./...
• Added TestXxxPerCallContentType for each of the six renderers.

Additional Context

The override path intentionally bypasses charset appending so callers retain full control of the header — for example, application/vnd.api+json already implies UTF-8 and shouldn't get a duplicate charset suffix.

[unrolled]

Looks good!