grab github permalink

This is a small Rust program that takes a Github permalink, like

https://github.com/torvalds/linux/blob/master/fs/autofs/dev-ioctl.c#L28-L36

and fetches the lines referred to in the #L${start}-L${finish} bit. In the case of that example link it will produce:

```c typedef int (*ioctlfn)(struct file *, struct autofssbinfo *, struct autofsdev_ioctl *);

static int check_name(const char *name) { if (!strchr(name, '/')) return -EINVAL; return 0; } ```