Natsulang

Solved a century hard problem: `if-elif-else`
+ Now you can write `if-elif-else` like: `if(a)(...)if(b)(...)(...)`

You needn't add a `;` after an `if` or a `for` `while`
+ like `{if(1)(2)(3)4}` is ok and will output 4

Fixed some details.

+ Added support of bytes-string and f-string. Now you can initialize a bytes-string like `b"233"`
+ Added update check. You can run `natsulang --check-updates` to check the newer versions of natsulang.
+ Added ignore first line. Now you can make a unix-based natsulang program like below:

!/usr/bin/env natsulang --ignore-header
Program contents ...

Added `jump` function that you all loved.

+ Fixed a strange bug.
The bug is: if you write the program like this: `{"1+2=3"}`, a strange error will occur.
+ Added support of bytes-string, but you need to write `bytes("string content","utf-8")` to define a bytes-string instead of `b"string content"`. This is not convenient, so it will be fixed later.