Fix image links with optional titles

[jay23606]

Summary

• Parse the image destination separately from an optional CommonMark title before embedding relative images.
• Preserve double-quoted, single-quoted, and parenthesized titles so Goldmark renders the image title attribute.
• Add regression coverage for titled and untitled relative images.

Fixes #60.

Validation

• go test -vet=off ./... passes. The repository baseline currently fails go vet because main.go:187 passes a redundant newline to fmt.Println; the same warning occurs on clean main.
• go build ./... passes.