Adding suspended_at attribute to github_user data source

[mattrobinsonsre]

Closes #978

Should be very low risk.

% go test -v ./... -run TestAccGithubUserDataSource
?   	github.com/terraform-providers/terraform-provider-github	[no test files]
=== RUN   TestAccGithubUserDataSource
=== RUN   TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error
=== RUN   TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error/with_an_anonymous_account
    data_source_github_user_test.go:40: anonymous account not supported for this operation
=== RUN   TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error/with_an_individual_account
    testing.go:541: Acceptance tests skipped unless env 'TF_ACC' set
=== RUN   TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error/with_an_organization_account
    testing.go:541: Acceptance tests skipped unless env 'TF_ACC' set
=== RUN   TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account
=== RUN   TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account/with_an_anonymous_account
    data_source_github_user_test.go:75: anonymous account not supported for this operation
=== RUN   TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account/with_an_individual_account
    testing.go:541: Acceptance tests skipped unless env 'TF_ACC' set
=== RUN   TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account/with_an_organization_account
    testing.go:541: Acceptance tests skipped unless env 'TF_ACC' set
--- PASS: TestAccGithubUserDataSource (0.00s)
    --- PASS: TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error (0.00s)
        --- SKIP: TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error/with_an_anonymous_account (0.00s)
        --- SKIP: TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error/with_an_individual_account (0.00s)
        --- SKIP: TestAccGithubUserDataSource/queries_an_existing_individual_account_without_error/with_an_organization_account (0.00s)
    --- PASS: TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account (0.00s)
        --- SKIP: TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account/with_an_anonymous_account (0.00s)
        --- SKIP: TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account/with_an_individual_account (0.00s)
        --- SKIP: TestAccGithubUserDataSource/errors_when_querying_a_non-existing_individual_account/with_an_organization_account (0.00s)
PASS
ok  	github.com/terraform-providers/terraform-provider-github/github	(cached)

[kfcampbell]

Thanks for doing this!