Impose a predefined order on the parameter documentation

- Parameter documentation is ordered according to the order of the formals, if possible
    - even with multiple `@usage` statements
- For `@param` statements with more than one parameter (comma-separated), the name of the
  first parameter is used
- The reordering is a separate process that runs just after processing parameter inheritance

Implementation:

- Add new optional parameter `overwrite` to `add_tag()`
    - Not sure if I missed existing functionality
- Refactor extraction of parameter names to a new function `get_documented_params`
    - Shouldn't we `strsplit` on `"\\s+,\\s+" instead?
- New function `fix_params_order`, similar to `process_inherit_params`
    - In new module `order-params.R`
    - Uses `get_documented_params` from `inherit-params.R` -- do we need an `@include` for the collation order?
- Tests

Also includes a workaround for duplicate entries in the `formals` tag (cf. https://github.com/klutometis/roxygen/commit/83d125dce50a072534988787d49ffe206d19b232#commitcomment-6742169)

Fixes #63.
Can you please matching indenting with the other tests?