nvm_format_version: add unit tests.
Create nvm_ensure_version_prefix, and make nvm_format_version actually format the version.
Add nvm_validate_implicit_alias
Adding nvm_print_implicit_alias
Adding nvm_print_implicit_alias error tests.
Adding nvm_print_implicit_alias success tests
Make `nvm alias` list implicit "stable" and "unstable" aliases when they're not already aliases.
Add support for `nvm ls stable` and `nvm ls unstable`
Support `nvm ls-remote stable` and `nvm ls-remote unstable`