ask: support `--default=<value>`, close #99 #675

Job	Run time
homebrew-macos-test	2s
login-shells-and-linting	4s
distro-test (ubuntu:latest)	10s
fresh-macos-test	4m 8s
distro-test (fedora:latest)	46s
distro-test (debian:latest)	20s
distro-test (alpine:latest)	5s
distro-test (manjarolinux/base)	24s
distro-test (opensuse/leap)	38s
distro-test (opensuse/tumbleweed)	19s
distro-test (kalilinux/kali-rolling)	16s
	7m 12s

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