fix: inconsistent directory name behaviour with nuxt-content

[ayZagen]

This pull request transforms asset directory names to match with nuxt-content logic.

Fixes #57

[davestewart]

Hey, just to let you know I took a look at this today.

My head's not quite in the game this week re. coding, but I'll probably make a couple of tweaks to the naming and add some tests then push pack to your branch in lieu of merging (if you can ensure I have edit permissions, that would be great).

Thanks for the work so far!

[ayZagen]

@davestewart sorry for the late reply. I gave access to you as you requested.

Thanks.