Add migration parameters

[krlohnes]

Does your PR solve an issue?

closes #3178

Is this a breaking change?

No, this is not a breaking change. The feature is gated by a comment annotation that surrounds the parts of the migration to be parameterized.

[abonander]

Sorry, I just don't like the approach here.

• Some users already don't like the reliance on environment variables.
• It can easy to forget to specify environment variables which would lead to an error at runtime.
• Users might expect more interaction with migrate!() or may even expect variables to be expanded at compile-time (or at least to have that option).
• It's impossible to programmatically set these variables without using env::set_var(), which is not thread-safe on most platforms.
• Using the regex crate to find the start and end of substitution-enabled sections is kind of overkill.

Because this is unlikely to be merged without significant changes, I'm just going to close. Let's take the discussion back to #3178 where I'll give my thoughts on what a proper solution should look like.