mutator 0.2.0

• Added first-class tinytest support. A package with an inst/tinytest
  directory is now auto-detected and its mutants are run in-process with
  pkgload::load_all() and tinytest::run_test_dir(), without an install per
  mutant, including coverage-guided test selection.
• Added a strategy argument to mutate_package() to override test-framework
  auto-detection. Accepted values are "auto" (the default), "testthat",
  "tinytest", "tinytest-installed", and "installed".
• Added a tinytest-installed strategy that installs each mutant and runs
  tinytest::test_package(). It is a fallback for packages whose in-process
  load diverges from an installed copy and unlike the generic
  installed-tests fallback it still supports coverage-guided selection.
• Reworked the README and configuration vignette, documenting how the test
  strategy is selected and when to override it.
• Fixed the per_file coverage backend to forward the package's
  tests/testthat.R harness arguments (notably any filter) to
  testthat::test_dir(), matching the record_tests backend.