feat: add filter option to sources (#1067)

* feat: add `source.filter` config

This allows the user to specify a `filter` function for each source,
like this:

```lua
-- don't show entries with kind "Text" from the "nvim_lsp" source

sources = {
{
  name = "nvim_lsp",
  filter = function(entry, ctx)
    local kind = types.lsp.CompletionItemKind[entry:get_kind()]

    if kind == "Text" then
      return true
    end
  },
}
```

By utilizing the `ctx` parameter, the user can also ignore certain
entries in certain contexts.

* fixup! feat: add `source.filter` config

* fixup! feat: add `source.filter` config

doc/cmp.txt

@@ -560,6 +560,31 @@ sources[n].group_index~
       })
     }
 <
+
+                                             *cmp-config.sources[n].entry_filter*
+sources[n].entry_filter~
+  `function`
+  A source-specific entry filter, with the following function signature:
+>
+  function(entry: cmp.Entry, ctx: cmp.Context): boolean
+<
+
+  Returning `true` will keep the entry, while returning `false` will remove it.
+
+  This can be used to hide certain entries from a given source. For instance, you
+  could hide all entries with kind `Text` from the `nvim_lsp` filter using the
+  following source definition:
+>
+  {
+    name = 'nvim_lsp',
+    entry_filter = function(entry, ctx)
+      return require('cmp.types').lsp.CompletionItemKind[entry:get_kind()] ~= 'Text'
+    end
+  }
+<
+  Using the `ctx` parameter, you can further customize the behaviour of the
+  source.
+
                                                                *cmp-config.view*
 view~
   `{ entries: cmp.EntriesConfig|string }`