improvements for 0.1.4 release

[kassambara]

Improvement from @IndrajeetPatil

• [x] adds visual regression tests
• [x] gets rid of NOTE in CRAN daily checks (https://cran.r-project.org/web/checks/check_results_ggcorrplot.html)
• [x] gets rid of warning with ggplot2 3.3.5
• [x] bumps needed version of ggplot2
• [x] adopt tidyverse convention for formatting DESCRIPTION file
• [x] spelling check
• [x] styling (code and NEWS.md)
• [x] adds Indrajeet Patil as a contributor