Add tags to .gitignore
Stylistic refactor to remove duplication
Refactor login to default to ~/.exercism
- Clean up login method by extracting to private methods
- Default to ~/.exercism config file while still supporting
  ~/.config/exercism config file

  Authors: @crymer11 and @zph
Merge pull request #45 from crymer11/master

Refactor `CLI.login` in the spirit of convention over configuration