Add Lua REPL mode and multiline edit function in the mode.

Typing "@repl" after Ctrl-t r, enter Lua REPL mode.
Typing "@exit" in REPL mode will return you to normal mode.

Note:
- the determination of continuation lines is not done automatically, and
  if the end of a line is \, it is determined to be a continuation
  instruction.
- In REPL mode, tio's main loop is blocked. (Ctrl-t q works.)

Example:
>> t = {1,2,3,4,5}
>> for _,v in ipairs(t) do\
>>     print(v,"\r")\
>> end

Implementation improvements:
- Add tty_init() and script_interp_init() in order to only once
  initialization.
- Fix script function to work without device_fd.

src/main.c

@@ -123,6 +123,12 @@ int main(int argc, char *argv[])
         socket_configure();
     }
 
+    /* Script interpreter init */
+    script_interp_init();
+
+    /* Initialize tty module once on program start */
+    tty_init();
+
     /* Spawn input handling into separate thread */
     tty_input_thread_create();