feat: Basic attachment support

src/awk/has.awk

@@ -0,0 +1,10 @@
+## src/awk/has.awk
+## Decide if VEVENT file has a specific field.
+##
+## @assign field: Field name
+
+# AWK program
+BEGIN              { FS = "[:;]" }
+/^BEGIN:VEVENT$/   { ins = 1 }
+/^END:VEVENT$/     { exit 1 }
+ins && $1 == field { exit 0 }