Impose a predefined order of 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
workaround for duplicates in the formals tag

cf. https://github.com/klutometis/roxygen/commit/83d125dce50a072534988787d49ffe206d19b232#commitcomment-6742169
.gitattributes to set merge strategy to "union" for NEWS.md
add NEWS item
fix indentation
update NEWS
assigning result of if statement
Revert "assigning result of if statement"

This reverts commit b55a0e714258a55b68fe7047ecfdb369d571b746.
Merge pull request #258 from krlmlr/63-param-order

Impose a predefined order on the parameter documentation