Support whitespace after +

Add support for whitespace after `+` when calling mixin.

Both `-` and `=` support whitespace after it, so why don't `+` also supports it. Then we can align them like this (by preference of the user)

```
- stmt
= var
+ mixin
  block
```
instead of

```
- stmt
= var
+mixin
  block
```
If you omit the brackets here (i.e. Use `\s*` instead of `(\s*)`) you won't be creating a capture group so you won't need to update all the other indexes.