Ability to set custom filename sanitizers

I wanted to specify custom sanitizer that mess with long filenames. Also it's useful when subdirs in filename are allowed:
```ruby
has_attached_file :avatar,
  restricted_characters:   /[&$+,:;=?@<>\[\]\{\}\|\\\^~%# ]/,
  filename_sanitizer:      ->(str, attachment) {
    attachment.cleanup_filename Pathname.new(str).cleanpath.to_s.gsub('../', '')
  }
```