Lost compatibility with iPython > 7

[Aar0wn]

Seems they have completely re-written the API for string and token based transformations. The issue seems to be with the lines involving the imports from 'prompt_tool.layout.utils.token'.

I've slightly modified and adapted your code to create a gruvbox theme and when I just updated to iPython 7 yesterday everything went to hell. I'll try to see if I can be of any help with the issue, hopefully I can learn some new things in the process.

Also, thank you for your work, it helped me to customize my prompts to my liking. I couldn't find any ipython themes to work from and yours did the job.