Add a Cask for Seashore.
user
```ruby
class Seashore < Cask
  url 'https://sourceforge.net/projects/seashore/files/seashore%20binaries/Seashore%200.5.1/Seashore.zip/download'
  homepage 'http://seashore.sourceforge.net/'
  version '0.5.1'
  content_length '6957082'
end
```
assistant