Mediumish does not support cascaded menus

[yesitsme007]

Hugo supports cascaded menus by defining a "parent" property. Unfortunately this is not supported by the Mediumish theme. It is simply ignored. It was quite a frustrating experience when I tried to define a menu with parent and it did not work. You have no clue if there is something wrong in the menu definition or why it does not work. It took me quite a while until I found out that it is the template which prevented it from working (and there seem to be many other templates with this restriction). Unfortunately the documentation of menus is poor in the Hugo docs which does not make thing easier.

I tried to make it working and submit this as PR #30

My use case is a quick navigation if you have many blog entries. I added an example to the example site:

[lgaida]

Hi thanks for your effort.

I remember that i tried to do this when i ported this theme from the original jekyll theme but gave up after a few tries. I was and still am a newbie to gohugo. The documentation of gohugo has also changed and new features were added. Looked like to much effort for me, since i just wanted to take advantage of the nice and clean css/design.
Can you add a description for this to the readme.md? I will try to recreate your example and merge your PR.