tail: added size suffixes handling

Rust beginner here, any comments welcome.
This could also be:
```rust
let exponent =
    if size_slice.len() > 0 {
        let mut has_suffix = true;
        let exp = match size_slice.char_at(size_slice.len() - 1) {
            'K' => 1u,
            'M' => 2u,
            'G' => 3u,
            'T' => 4u,
            'P' => 5u,
            'E' => 6u,
            'Z' => 7u,
            'Y' => 8u,
            // 'b' would go here if it's valid
            _ => {
                has_suffix = false;
                0u
            }
        };
        if has_suffix {
            size_slice = size_slice.slice_to(size_slice.len() - 1);
        }
        exp
    } else {
        0u
    };