realpath and relpath implementation

Realpath (#315) provides set of features similar to the plain Debian implementation. GNU options aren't supported yet.
Relpath (#316) options are similar to the old one FreeBSD relpath package, since no coreutils implementation currently providing this command.
I would prefer this to be something like:
```rust
let from =
    if opts.free.len() > 1 {
        Path::new(opts.free.get(1).as_slice())
    } else {
        std::os::getcwd()
    };
```