tail: added size suffixes handling

Rust beginner here, any comments welcome.
You could make this:
```rust
let base =
    if size_slice.len() > 0 && size_slice.char_at(size_slice.len() - 1) == 'B' {
        size_slice = size_slice.slice_to(size_slice.len() - 1);
        1000u
    } else {
        1024u
    };
```