`ion-cli beta schema validate` cannot read an Ion file as an ISL `document`

[popematt]

Given an Ion file such as

a
b
c

the ion-cli beta schema validate command will perform separate validations on each value in this file instead of reading and validating the file as a document, even though a document is supposed to represent a stream of top-level Ion values.

There may be situations where you do want to validate each top-level value separately, so it might be necessary to have an option to indicate how the input should be processed (as individual values or as a stream).