Add explicit support for UUIDs

## I'm submitting a...

[ ] Bug report  

[x] Feature request

## Current behavior

When I add a column with a type of `uuid`, I get the following warning:

```
[WARN] Using unknown data type UUID
```

## Expected behavior

I would expect no warning message as long as I have the `pgcrypto` extension installed.

## Minimal reproduction of the problem with instructions

Add a column with a type of `uuid`.

## What is the motivation / use case for changing the behavior?

Personally, I'm a fan of having no warnings in my project. I believe when warnings start piling up that aren't problems, developers begin to ignore them, which causes real issues to be missed. In this case, the `uuid` type is valid, but `db-migrate` doesn't recognize it.

## Environment

```
db-migrate version: 0.10.2
db-migrate-pg: 0.2.5
db-migrate-plugin-babel: 1.0.0

Additional information:
- Node version: 9.2
- Platform: Linux (via Docker)
```

Others:

I believe this issue originally surfaced in #34. I do think allowing pass-through column info is a good idea, but I don't feel like UUIDs should produce warnings.

---
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