Add an importer for just Slack's user export

.meteor/packages

.meteor/versions

packages/rocketchat-i18n/i18n/en.i18n.json

...s/rocketchat-importer-slack-users/.npm/package/.gitignore

packages/rocketchat-importer-slack-users/.npm/package/README

...hat-importer-slack-users/.npm/package/npm-shrinkwrap.json

packages/rocketchat-importer-slack-users/client/adder.js

packages/rocketchat-importer-slack-users/info.js

packages/rocketchat-importer-slack-users/package.js

packages/rocketchat-importer-slack-users/server/adder.js

packages/rocketchat-importer-slack-users/server/importer.js

packages/rocketchat-importer/server/classes/ImporterBase.js