The new component: ex.lsp.null_ls

[vladimir-popov]

The new component shows names of the null-ls sources according to the specified query.
By default, it shows names of all sources actual to the current buffer. All duplicated names are merged.

sections = {
  lualine_a = {
    {
      'ex.lsp.null_ls',

      -- The table or function that returns the table with the source query.
      -- By default it shows only actual sorces. To show all registered sources
      -- you can use just empty table:
      -- query = {}
      query = function()
        return { filetype = vim.bo.filetype }
      end,

      -- The string separator between names
      source_names_separator = ',',

      -- The color for the disabled component:
      disabled_color = { fg = 'grey' }

      -- The color for the icon of the disabled component:
      disabled_icon_color = { fg = 'grey' }
    }
  }
}

No one source: 
The jq and the spell sources are active for the current buffer:

Closes #25