Return a nonzero exit code from nvm_rc_version when no .nvmrc found
If there's no "default" specified, fall back to an .nvmrc file.
Unit tests: add `nvm_has`
Unit tests: add `nvm_find_up`
Use `nvm_normalize_version` in `nvm_version_greater`
Use `nvm_version_greater` in `nvm_binary_available`
Adding nvm_num_version_groups
Use `nvm_num_version_groups` in `nvm_ls`