Diff command should imply validation

# Description
Currently, running the `convection diff` command does not validate the template for a given stack and can suppress errors that come up when converging. It would be nice if the `diff` command could also `validate` the stack that you're `diff`ing.

## Expected behavior
`Diff`ed stacks are also `validate`d.

## Observed behavior
`Diff`ed stacks are *not* `validate`d. 😛 