Use visitor pattern in planemo dependency_script

The initial `planemo dependency_script` implementation https://github.com/galaxyproject/planemo/commit/f798c7e29b2276ce68b828e72fc6a6460c73792b defines a `.to_bash()` method on the action classes in `planemo/shed2tap/base.py`. This has downsides such as mingling code into the `shed2tap` folder.

In discussion on #303 @jmchilton wrote: _I was thinking implementing a [visitor pattern](https://en.wikipedia.org/wiki/Visitor_pattern) for ruby/bash conversion - but `to_bash` or `to_ruby` will be find also._