Add version info to 'sandbox status'

[joshuaauerbachwatson]

This change allows the developer to know the version of the sandbox support, which is distinct from the doctl version.

Because doctl sandbox status relies on error returns for common cases (not installed, not at the right version, not connected) I decided on the following.

If you issue doctl sandbox status and the status is connected, you get all the information, including version:

> doctl sbx status
Connected to function namespace 'fn-5a...' on API host 'https://faas-....doserverless.co'
Sandbox version is 3.0.10-1.2.1

If the status is other than connected, you just get the error as before.

However, as long as sandbox support is installed, you can always get version information by specifying a flag. The output differs according to whether your sandbox support is up to date.

> doctl sbx status --version
3.0.10-1.2.1

if up-to-date or

> doctl sbx status --version
Current: 3.0.9-1.2.1, required: 3.0.10-1.2.1

if an upgrade is required.

[scotchneat]

LGTM