**Breaking changes**:
* Move `_strip`, `_strip_end` before `_modify` (89)
* Existing patch files may need updating to refer to the stripped
versions of names being modified
* Allow `_derive` to rename derived peripherals, optionally specify a new base
address and description (118)
* If registers were being copied and modified, use `_copy` instead of
`_derive` for those peripherals.