Convert CLI to urfave v2

Followed the Migration Guide at https://cli.urfave.org/migrate-v1-to-v2/
The major changes not pointed out in the migration guide are:
- context.Args() no longer produces a []slice, so context.Args().Slice()
  in substitued
- All cli.Global***** are deprecated (the migration guide is somewhat
  unclear on this)

Signed-off-by: Derek Nola <derek.nola@suse.com>

Vendor in urfave cli/v2

Signed-off-by: Derek Nola <derek.nola@suse.com>

Fix NewStringSlice calls

Signed-off-by: Derek Nola <derek.nola@suse.com>

vendor/github.com/urfave/cli/v2/Makefile

@@ -0,0 +1,26 @@
+# NOTE: this Makefile is meant to provide a simplified entry point for humans to
+# run all of the critical steps to verify one's changes are harmonious in
+# nature. Keeping target bodies to one line each and abstaining from make magic
+# are very important so that maintainers and contributors can focus their
+# attention on files that are primarily Go.
+
+GO_RUN_BUILD := go run internal/build/build.go
+
+.PHONY: all
+all: generate vet test check-binary-size gfmrun yamlfmt v2diff
+
+# NOTE: this is a special catch-all rule to run any of the commands
+# defined in internal/build/build.go with optional arguments passed
+# via GFLAGS (global flags) and FLAGS (command-specific flags), e.g.:
+#
+#   $ make test GFLAGS='--packages cli'
+%:
+	$(GO_RUN_BUILD) $(GFLAGS) $* $(FLAGS)
+
+.PHONY: docs
+docs:
+	mkdocs build
+
+.PHONY: serve-docs
+serve-docs:
+	mkdocs serve