Can't read books that added by relative path

I added book by relative path.

    $ honyomi add relative/path/to/book
    
And launch web interface.

    $ honyomi web

Search the book and click to raise Errno::ENOENT.
    
    Errno::ENOENT at /v/1
    No such file or directory @ rb_file_s_stat - relative/path/to/book

I added `filename = File.expand_path(filename)` in `Honyomi::Core#add` to fix it. Is there better way?