feat: support number of context lines

The default number of context lines is 3 and defined in
`udiff.DefaultContextLines`

Fixes: #5

export.go

@@ -5,6 +5,6 @@ type UnifiedDiff = unified
 
 // ToUnifiedDiff takes a file contents and a sequence of edits, and calculates
 // a unified diff that represents those edits.
-func ToUnifiedDiff(fromName, toName string, content string, edits []Edit) (UnifiedDiff, error) {
-	return toUnified(fromName, toName, content, edits)
+func ToUnifiedDiff(fromName, toName string, content string, edits []Edit, contextLines int) (UnifiedDiff, error) {
+	return toUnified(fromName, toName, content, edits, contextLines)
 }