resource/github_team: Expose slug (and document when to use it)

[radeksimko]

Fixes #84

I'm labelling this as enhancement, as it's technically not a provider bug, but rather API bug, or undocumented behaviour.

The API silently ignores invalid team names, which can be either:

• teams which simply don't exist
• teams which don't have access to the repository
• valid team names, instead of slugs

TF_ACC=1 go test ./github -v -run=TestAccGithubBranchProtection_ -timeout 120m
=== RUN   TestAccGithubBranchProtection_basic
--- PASS: TestAccGithubBranchProtection_basic (5.68s)
=== RUN   TestAccGithubBranchProtection_teams
--- PASS: TestAccGithubBranchProtection_teams (10.49s)
=== RUN   TestAccGithubBranchProtection_emptyItems
--- PASS: TestAccGithubBranchProtection_emptyItems (4.38s)
=== RUN   TestAccGithubBranchProtection_importBasic
--- PASS: TestAccGithubBranchProtection_importBasic (3.88s)
PASS
ok  	github.com/terraform-providers/terraform-provider-github/github	24.470s

TF_ACC=1 go test ./github -v -run=TestAccGithubTeam_ -timeout 120m
=== RUN   TestAccGithubTeam_basic
--- PASS: TestAccGithubTeam_basic (3.49s)
=== RUN   TestAccGithubTeam_slug
--- PASS: TestAccGithubTeam_slug (1.22s)
=== RUN   TestAccGithubTeam_hierarchical
--- PASS: TestAccGithubTeam_hierarchical (2.56s)
=== RUN   TestAccGithubTeam_importBasic
--- PASS: TestAccGithubTeam_importBasic (1.25s)
PASS
ok  	github.com/terraform-providers/terraform-provider-github/github	8.545s

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