Issue #139: Username Lookup Fails for User IDs that Start with W.

[jameskbride]

In This PR

• [x] Minor update to the .gitignore to exclude Intellij project files. (I can remove this if it is an annoyance.)
• [x] Added another version of Slack.Lookups.lookup_user_name/2 which handles user ids that start with 'W', per the documentation. This change addresses https://github.com/BlakeWilliams/Elixir-Slack/issues/139

[jameskbride]

Closing this in favor of the fixes in https://github.com/BlakeWilliams/Elixir-Slack/pull/188