v0.3.0

• New ?ignore-before-r parameter to ignore optional dependencies that
  need a newer R version (#243).

• New ?ignore parameter to ignore an optional dependency.

• Allow specifying downstream package parameters with the package=?param
  syntax.

• The $update() operation now works better for any:: refs, and we
  always install the version we planned for.

• System requirement installation is now more robust and works for
  Unix shell expressions (#347).

• Make system dependency installation more robust
  (r-lib/pak#347).

• CRAN-like resolution is more robust now if a repository is missing
  the usual metadata.

• The lock file is pretty JSON now.

• pkgdepends now does not return tibbles, but simple data frames.
  They are still printed concisely as long as the pillar package is loaded.

• pkgdepends now handles all version requirement types properly:
  '<', '<=', ==, >=, >.