Add basic support for running external programs

.gitignore

@@ -1,2 +1,2 @@
 /target
-inp.txt
+*.txt

src/main.rs

@@ -140,6 +140,13 @@ impl Program {
         builder
     }
 
+    fn run_external(&mut self, filename: String) {
+        // Read contents of the provided file and construct a symbolic Program from it
+        let contents = fs::read_to_string(filename).expect("Something went wrong reading the file");
+        let mut prog = Program::from_string(contents);
+        prog.run();
+    }
+
     fn parse(&mut self, instruction: &String) {
         // Opcode is the first character, arguments are everything after the first char
         let opcode = instruction.chars().collect::<Vec<char>>()[0];
@@ -154,6 +161,7 @@ impl Program {
                 'd' => println!("{}", eval::do_math(self.args_or_vars(&arguments), '/')),
                 'l' => self.add_var(&arguments),
                 'f' => self.add_func(&arguments),
+                'i' => self.run_external(arguments),
                 _ => panic!("SyntaxError at opcode {}: Unknown opcode {}", self.pc, opcode),
             }
         }