Add option for LTO

[devonhollowood]

Sometimes it is interesting to see and discuss the generated assembly for code with LTO turned on. For example, this code:

extern crate itertools;
use itertools::Itertools;

pub fn foo(x: u32, y: u32) -> bool {
    vec![x, y].into_iter().dedup().count() == 1
}

gets compiled down to just x == y when compiled with --release and lto = true, which is pretty wild. But there's no way to share or discuss that easily online, since this site doesn't allow for LTO and godbolt's site doesn't allow for external crate imports. So, it would be nice if this site allowed the users to turn on LTO.

If the top bar is getting too crowded, maybe it makes sense to have a drop-down box where one can specify additional, less-used compiler options? I'm not sure how best to go about this from a UI standpoint but it would be nice to have.

[shepmaster]

/cc @Chandlore and #272