Plot-playground

- Added storytelling-like slope plot.

- Google Colaboratory and Kaggle Kernel support.
- Fix date column bug. https://github.com/simon-ritchie/plot_playground/issues/7

- Added document.
- Fix cliping path bug
- Default style tweak

- Fixed error in HTML.
- Changed so that it can be used for data frame of month and year.
- Build script added.

Added a storytelling-like simple line plot.