Allow sorting items by source

[pbogut]

I've marked it as Work In Progress. Initially it sorts items just fine, but when I keep typing it starts to mixed up and it looks like items in in sort function are missing source_priority. I need to figure out what is happening there.
It was being dropped in util.addCompletionItems(), I've added source priority there to fix that.

It allows items to be sorted by source (issue #273).

It works similar to items_priority option. I extended sort function to check for source_priority and sort by it when available.
I had to add source_priority to completion items, and I think the only place where we have access to both is when completion items are fetched from external / internal sources, so I added change there. Source priority defaults to 1 if its not set.

Configuration:

vim.g.completion_source_priority = {
  lsp = 10,
  ['vim-dadbod-completion'] = 9,
  snippet = 5,
  ts = 7,
  buffers = 1,
}

or vim syntax:

let g:completion_source_priority = {
  \  'lsp': 10,
  \  'vim-dadbod-completion': 9,
  \  'snippet': 5,
  \  'ts': 7,
  \  'buffers': 1,
  \  }