Initial commit: alpha status (0.4.0)

rootca and subca functionality are proof of concept. CLI structure is
mostly in place.
feat(src): add .gitignore
feat(src): add README.md
feat(cli): change "create" command to "sign"
docs(cli): reword help, change "create" cmd to "sign"
fix(main.subca): configuration naming when parent is root

Configuration filename format is different for referencing root vs
referencing another subca, so change accordingly when running command.