New interpretation/Visualization API

[lorenzoh]

Implements the ideas in #154.

To-Dos:

• [x] abstract types and interfaces
• [x] backends
  • [x] Text
  • [x] Makie
• [x] High-level functions for use with
  • [x] LearningMethods
  • [x] Learners
• [x] default implementation for WrapperBlocks
• [x] Bounded block
• [x] new text-based display for learning rate finder
• [x] lazy-load Makie.jl with Requires.jl. This reduces using time from 22 secs. to 17 on my machine (10 of which seem to come from Flux.jl)
• [x] tests
• [x] update documentation
  • [x] use show instead of plot everyhwere
  • [x] update "How to visualize"
• [x] update CHANGELOG

[lorenzoh]

Depends on https://github.com/ronisbr/PrettyTables.jl/pull/143

[lorenzoh]

Example of high-level visualization derived from a learning method. Also, custom table printing formats supported in ShowText.