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,
  \  }