cmd/cue/cmd: allow lossy conversion by default and add --strict option

Applies to JSON Schema now, but generallly applicable for
other conversions.

Change-Id: Ie6abe0ea0daf2d2ede1850a16cd2c1d831fb7f13
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5649
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/jsonschema/jsonschema.go b/encoding/jsonschema/jsonschema.go
index 09e4173..5f5791b 100644
--- a/encoding/jsonschema/jsonschema.go
+++ b/encoding/jsonschema/jsonschema.go
@@ -57,7 +57,8 @@
 type Config struct {
 	PkgName string
 
-	ID string // URL of the original source, corresponding to the $id field.
+	// ID sets the URL of the original source, corresponding to the $id field.
+	ID string
 
 	// JSON reference of location containing schema. The empty string indicates
 	// that there is a single schema at the root.
@@ -81,5 +82,9 @@
 	// - selection and definition of formats
 	// - documentation hooks.
 
+	// Strict reports an error for unsupported features, rather than ignoring
+	// them.
+	Strict bool
+
 	_ struct{} // prohibit casting from different type.
 }