Support for new scratch org Features (and possibly Org Preferences)

[Chris Sorrows]

The list of Features displayed when creating a new scratch org doesn't list all of the ones that Salesforce lists in their doc at:

https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_scratch_orgs_def_file_config_values.htm

For example, a new one I was trying to test with (ExternalSharing) is not listed, although it's available for use in a definition file.

There may also be new values for Org Preferences, but I didn't check those as I rarely use them.

[Scott Wells]

Issue #985 was marked as a duplicate of this issue.

[Scott Wells]

Issue #984 was marked as a duplicate of this issue.

[Scott Wells]

Issue #983 was marked as a duplicate of this issue.

[Scott Wells]

Forgot to resolve this, but should have been included a few builds back. I'll also be updating for the new Summer '18 features/org preferences in next week's build.