Add options to migrator Validate functions + option for TargetVersion

[brandur]

This one in response to #1257. It's useful to validate the current state
of the database's migrations, but you may not be fully on the latest
version and could get an error. The included example is for Pro v0.24.0
which includes two migrations: one to be rolled out before deploy and
one to be rolled out after. Validate would fail until the post-deploy
migrations were run, which isn't flexible enough.

Here, have Validate functions take a ValidateOpts similar to the
MigrateOpts taken by the Migrate functions. ValidateOpts includes
TargetVersion, which lets you specify a maximum version bound.

Notably, this is a breaking change in that we add a new parameter to
Validate. I'm a little on the fence about this, but doing it via
breaking (1) avoids an extra pair of functions that pollute the API and
need testing, (2) should be mostly low impact because I doubt all that
many people are using Validate, and (3) the fix is super easy in that
you just need to add a nil in the last parameter place if you don't
want to use any new options.

Fixes #1257.