Add support for writing spreadsheets with cells containing rich text

[edsko]

I'm working on a PR for this. Opening an issue so that you are aware it's in the works.

[edsko]

Ok, preliminary version of this is done; I've tried to keep the changes to the existing code minimal, so there are two completely new modules (https://github.com/edsko/xlsx/blob/pr/support-rich-text/src/Codec/Xlsx/SharedStringTable.hs and https://github.com/edsko/xlsx/blob/pr/support-rich-text/src/Codec/Xlsx/RichText.hs) and only a handful of changes to the rest, the main one being

data CellValue = CellText   Text
               | CellDouble Double
               | CellBool   Bool
               | CellRich   [RichTextRun]  -- This line is new
               deriving (Eq, Show)

I will submit this as a PR proper once I've used this a bit more, I'll probably end up polishing it a bit more.

[qrilka]

closed by #36