Add code coverage reporting and update README badges

- Integrate codecov for test coverage reporting - Add codecov and
R-CMD-check badges to README - Update roadmap and documentation to
reflect coverage goals - Add codecov.yml configuration file - Enable
tests in tests/testthat.R - Update example images in man/figures

DESCRIPTION

@@ -2,7 +2,7 @@ Type: Package
 Package: visualizeR
 Title: What a color! What a viz!
 Version: 1.0
-Authors@R: 
+Authors@R:
     person("Noblet", "Guillaume", , "gnoblet@zaclys.net", role = c("aut", "cre"))
 Maintainer: Guillaume Noblet <gnoblet@zaclys.net>
 Description: It basically provides colors as hex codes, color palettes,
@@ -10,9 +10,9 @@ Description: It basically provides colors as hex codes, color palettes,
 License: GPL (>= 3)
 URL: https://github.com/gnoblet/visualizeR,
     https://gnoblet.github.io/visualizeR/
-Depends: 
+Depends:
     R (>= 4.1.0)
-Imports: 
+Imports:
     checkmate,
     dplyr,
     forcats,
@@ -28,7 +28,8 @@ Imports:
     tidyr,
     viridisLite,
     waffle
-Suggests: 
+Suggests:
+    covr,
     knitr,
     rio,
     rmarkdown,
@@ -36,7 +37,7 @@ Suggests:
     testthat (>= 3.0.0),
     vdiffr,
     mockery
-VignetteBuilder: 
+VignetteBuilder:
     knitr
 Config/testthat/edition: 3
 Encoding: UTF-8