Support GNU case ranges

[00xc]

Case ranges are a GNU extension that allow matching a number of consecutive cases, like so:

#include <stdio.h>
int main() {
	int v = 4;

	switch (v) {
		case 0 ... 4: puts("Between 0 and 4"); break;
		case 5: puts("5"); break;
		default: puts("something else"); break;
	}

	return 0;
}

This will be compiled without errors with gcc -std=gnu11 range.c. However, when parsing with lang-c, this results in a SyntaxError:

use lang_c::driver::{Config, parse};

fn main() {
   let config = Config::with_gcc();
    let p = parse(&config, "range.c");
    if let Err(e) = p {
        println!("{}", e);
    }
}

Output:

syntax error: unexpected token at line 733 column 11, expected '[_a-zA-Z]'

[vickenty]

Thanks!