minor fixes to documentation

[IndrajeetPatil]

This PR addresses a few minor issues-

1. In all README and roxygen examples, the argument outline.color was written as outline.col, which created warnings in RStudio scripts about the partial matching of arguments. Fixed that.
2. The resolution of figures in README was poor and text in figures was not legible in README available on CRAN. The update fixes that using dpi of 300 and output width of 100%.
3. It is advised not to include the date in the DESCRIPTION file (jrowen/rhandsontable#83). Removed the date accordingly (This was also creating problems for a stricter version of devtools::check()).
4. Styled the code in tidyverse style guide (both in R script and README file).
5. Added spelling tests to make sure no spelling error fall through the cracks.
6. Bumped up the package version to highlight that this is the development version. Added a few more badges to README to convey the same thing.
7. The digits argument (introduced in #12) wasn't working properly (IndrajeetPatil/ggstatsplot#93). This is now fixed. Also added an example to show that this works.

[IndrajeetPatil]

Here is an example with the bug in digits argument fixed-

library(ggcorrplot)
#> Loading required package: ggplot2

# Changing number of digits for correlation coeffcient
ggcorrplot(cor(mtcars), 
           type = "lower", 
           insig = "blank", 
           lab = TRUE, 
           digits = 3)

^{Created on 2018-12-17 by the reprex package (v0.2.1)}

[kassambara]

merged thanks