Enable map configurations (config v2).

[asottile]

This resolves #414

I've also added the following to aid in migration:

• pre-commit migrate-config
  • attempts to intelligently upgrade the configuration to the new map format
• pre-commit autoupdate - by default calls migrate_config(...)

For now I have opted to not issue a deprecation warning, I'll follow up with that in a later release.

[asottile]

This is mostly ready for review, I need to make migrate-config handle yaml that looks like this:

---
-   repo: ...
    ...

And should probably handle this case better:

current

+repos:
 # this is a pre-commit config
 -   repo: ...
     ...

better

 # this is a pre-commit config
+repos:
 -   repo: ...
     ...

[asottile]

Now handles both of those cases!

[chriskuehl]

nice!

[chriskuehl]

weird blank line?

[asottile]

this matches the other ones in the file, I plan to add more keys here so it'll look more natural eventually