Prepare for CRAN

[coolbutuseless]

Misc Fixes

• [x] Check for strlen() prior to using hex2nibble - user may be using short colour codes! e.g. #abc
• [x] Support short colour codes like #abc and #abcf
• [x] Support for named R colours? Use implementation from {colourlookup}
• [x] Figure out how to clearly show the supported feature grid
• [x] Do a feature comparison table with png::readPNG/writePNG() ?
• [x] README - better suite of examples

CRAN

• [x] desc::desc_normalize()
• [x] usethis::use_tidy_description()
• [x] rhub checks: clang-asan, valgrind, rchk
• [x] winbuilder checks
• [x] macbuilder checks
• [x] update copyright info
• [x] checkhelper::find_missing_tags()
• [x] checkhelper::check_as_cran()
• [x] urlchecker::url_check()
• [x] Check all exported functions have @examples
• [x] compile with -Wconversion
• [x] Fix version number. Update/consolidate NEWS.md