dev: upgrade to roxygen2 v4
Bump version for development
Improve package documentation
Add URL for additional mirror registrations
Add new package options for extra registrations
Check extra directories for registrations
doc: NB extra registrations will override base ones
doc: xform_brain typos
Teach xform_brain to handle character vector inputs
tests to confirm we can use extra registrations
Merge branch 'feature/extra-reg-dir'

* closes #15
* feature/extra-reg-dir:
  tests to confirm we can use extra registrations
  Teach xform_brain to handle character vector inputs
  doc: xform_brain typos
  doc: NB extra registrations will override base ones
  Check extra directories for registrations
  Add new package options for extra registrations
  Add URL for additional mirror registrations