feat: allow multiple hlgroups inside one column

[Bekaboo]

This PR adds the ability to set multiple hlgroups inside one column.

Related to #212

Before:

After:

To achieve the effect of the second screenshot, one has to setup oil as follows:

local permission_hlgroups = {
  ['-'] = 'NonText',
  ['r'] = 'DiagnosticSignWarn',
  ['w'] = 'DiagnosticSignError',
  ['x'] = 'DiagnosticSignOk',
}

oil.setup({
  columns = {
    {
      'permissions',
      highlight = function(permission_str)
        local hls = {}
        for i = 1, #permission_str do
          local char = permission_str:sub(i, i)
          table.insert(hls, { permission_hlgroups[char], i - 1, i })
        end
        return hls
      end,
    },
    { 'size', highlight = 'Special' },
    { 'mtime', highlight = 'Number' },
    {
      'icon',
      default_file = icon_file,
      directory = icon_dir,
      add_padding = false,
    },
  },
  win_options = {
    number = false,
    relativenumber = false,
    signcolumn = 'no',
    foldcolumn = '0',
    statuscolumn = '',
  },
})

[Bekaboo]

@stevearc Any improvements I can do to get this into master?

[Bekaboo]

Thanks for the type fix, I was just trying to figure out how to fix it.

[stevearc]

Thanks for the PR!

Sorry for the wait, I'm stretched pretty thin at the moment