Placeholder support for snippet engines (#76)

- Add support for placeholders in snippet insertion !
  None: placeholders are automatically set when using a bundled snippet engine.
  - Add `enable_placeholders` option (see |neogen-configuration|)
  - Add `placeholders_text` option (see |neogen-configuration|)

Co-authored-by: danymat <d.danymat@gmail.com>

doc/neogen.txt

@@ -101,6 +101,27 @@ Neogen provides those defaults, and you can change them to suit your needs
 
       -- Use a snippet engine to generate annotations.
       snippet_engine = nil,
+
+      -- Enables placeholders when inserting annotation
+      enable_placeholders = true,
+
+      -- Placeholders used during annotation expansion
+      placeholders_text = {
+          ["description"] = "[TODO:description]",
+          ["tparam"] = "[TODO:tparam]",
+          ["parameter"] = "[TODO:parameter]",
+          ["return"] = "[TODO:return]",
+          ["class"] = "[TODO:class]",
+          ["throw"] = "[TODO:throw]",
+          ["varargs"] = "[TODO:varargs]",
+          ["type"] = "[TODO:type]",
+          ["attribute"] = "[TODO:attribute]",
+          ["args"] = "[TODO:args]",
+          ["kwargs"] = "[TODO:kwargs]",
+      },
+
+      -- Placeholders highlights to use. If you don't want custom highlight, pass "None"
+      placeholders_hl = "DiagnosticHint",
   }
 <
 # Notes~
@@ -159,12 +180,23 @@ Feel free to submit a PR, I will be happy to help you !
 We use semantic versioning ! (https://semver.org)
 Here is the current Neogen version:
 >
-  neogen.version = "2.5.0"
+  neogen.version = "2.6.0"
 <
 # Changelog~
 
 Note: We will only document `major` and `minor` versions, not `patch` ones.
 
+## 2.6.0~
+  - Add support for placeholders in snippet insertion !
+    - Add `enable_placeholders` option (see |neogen-configuration|)
+    - Add `placeholders_text` option (see |neogen-configuration|)
+
+  Example placeholders:
+>
+ --- [TODO:description]
+ ---@param param1 [TODO:parameter] [TODO:description]
+ function test(param1) end
+<
 ## 2.5.0~
   - Ruby: Add support for `tomdoc` (http://tomdoc.org)
 ## 2.4.0~
@@ -202,6 +234,15 @@ To use a snippet engine, pass the option into neogen setup:
 Some snippet engines come out of the box bundled with neogen:
 - `"luasnip"` (https://github.com/L3MON4D3/LuaSnip)
 
+If you want to customize the placeholders, you can use `placeholders_text` option:
+>
+ require('neogen').setup({
+   placeholders_text = {
+     ['description'] = "[description]",
+   }
+ })
+<
+
 # Add support for snippet engines~
 
 To add support to a snippet engine, go to `lua/neogen/snippet.lua`.