feat(dashboard): allow text keys and preset.header to be functions

[dsully]

Description

Add the ability for text keys and preset.header to be functions.

This allows me to call this:

preset = {
    header = function()
        local heatmap = require("git-dashboard-nvim").heatmap()

        -- This triggers git-dashboard-nvim's highlights
        vim.cmd.doautocmd("FileType")

        return { table.concat(heatmap, "\n") }
    end,
    ...
}