Nelsie

New features

* New PDF renderer. Nelsie now produce smaller slides faster when raster images are used
* Rendering slides & image preprocessing in parallel

Fixes

* Fixed 19

New features

* Subslides insertion (`.slide_at()`)
* Custom code syntaxes and code color themes

New features

* text itself can be `InSteps`
* `ListBox` helper added
* `default_code_language` added (by fgelm01)
* `default_theme` renamed to `default_code_theme`

New features

* verbose level & progress bar