add2ggplot 0.3.0

• Add cran badge.
• Fix image path, update disclaimer, update readme framework.
• Fix the invalid url https://en.wikipedia.org/wiki/w._E._B._Du_bois
• Update the year of the license.
• Use 'donttest' instead of 'dontrun' into the functions.
• Add '@return' into the function metadata.
• Add examples for all exported functions.
• Add depends for some functions.
• Move used files into 'inst/extdata'.
• Add code of conduct
• Fix invalid urls.
• Fix mis-spelling.
• Drop invalid urls.
• Fix title in the tite case.
• Add cran comments.
• Added the function of theme_classic2, update coc in README.Rmd.