* allow customizing decrease delay of rgb heatmap * rename rgb typing heatmap decrease delay variable * address review comments * nix-shell: add clang-tools required for formatting the C code * heatmap: use real timer to track decrement rate * heatmap: fix ifndef var name typo * heatmap: add docs * Update docs/feature_rgb_matrix.md Co-authored-by: Drashna Jaelre <drashna@live.com> Co-authored-by: Drashna Jaelre <drashna@live.com>