Logging

[josephbuchma]

I see only 2 things that should be logged from user's point of view:

• Migration progress (what version is currently being applied)
• Errors

Latter can be logged by user (e.g. journey CLI), as errors are returned immediately.
Adding logging interface only to log migration progress seems like overkill.

In #3 I implemented ability to add pre/post migration hooks. Hook has following signature:

func(f file.File) error

So you can add "pre" hook like this:

func logMigrationHook(f file.File) error {
    examplelogger.Infof("Applying version %d", f.Version)
    return nil
}

(see PR for details on hooks api)

That pretty much solves the issue without extra bloating.

Except, maybe, debugging. Debug log is not something user would like to face, so we can simply bake any logger in, and make option to enable it.

Your thoughts ?