CRAN edits

[polettif]

These are the changes I made to bring the package a bit closer to a CRAN release (see #11)

It does pass R CMD check now and I fixed other stuff, see below. However, there might be licensing issues with chroma.js if it's put on CRAN, I can't really tell. Also, CRAN might demand further changes in the docs.

In short @jiho: Feel free to use these edits but I'm afraid there might still be some back and forth necessary with CRAN to get it accepted. The package is usable as is, so it might not be worth the hassle...

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Major changes:

• fixed URLs with urlchecker::urlcheck(). However, some links are no longer accessible, I changed the URLs to \code() This will probably not get accepted by CRAN.
• Reworded package description. There are a lot of acronyms in there and CRAN might demand changes, hard to tell.
• Move ggplot2 to imports as too many functions use it to catch it
• document scicol and scicol_info data

Removed development features:

• remove unfinished vignette
• remove scale_vsup code

Bugfixes:

• remove "CSS but not R" color name from parse_hex
• use && and || in conditions (insted of & and |)
• change tests to show that alpha and rgba are not reversible (these colors are not found via chroma.hex() but chroma.css(), added TODO note to code)
• move failing alpha() example with recycled vectors to dontrun

Housekeeping:

• update DESCRIPTION tags and roxygen version
• bump R version (stringr depends on R >= 3.6)
• add @noRd to v8 functions
• ignore .DS_Store