Add --no-add-history flag

Flag prevents the input stream from saving terms to ~/.scryer_history
when set. Use the flag when running tests to increase test isolation.

src/machine/args.rs

@@ -0,0 +1,16 @@
+use std::collections::BTreeSet;
+use std::env;
+
+#[derive(Debug)]
+pub struct MachineArgs {
+    pub add_history: bool,
+}
+
+impl MachineArgs {
+    pub fn new() -> Self {
+        let args: BTreeSet<String> = env::args().collect();
+        Self {
+            add_history: !args.contains("--no-add-history"),
+        }
+    }
+}