Add a preference to allow for custom colours

[dsample]

In the intent-rainbow plugin, there is a preference for a list of colours. When specified, the array would be processed on a loop, rather than the default colour list.

When processing a CSV file with a varying length of content with one column, it's hard to find the columns which line up, the further you scroll along the row with only a small number of different colours.

[mechatroner]

indent-rainbow uses a different technology, instead of syntax defintion files it relies on VSCode setDecorations API method. As I understand this method offers greater flexibility at the cost of performance. I didn't check but I suspect it won't work for large CSV files.
Still if VSCode would support dynamic generation of syntax files it would be possible to increase the number of colors, but I don't think this could be implemented any time soon.