Preconditions

[jzabroski]

From #664 , @DustinVenegas wrote a good suggestion:

I believe a fluent expression syntax for Preconditions might be useful here instead. That chaining methodology might be less invasive and more flexible in the long-run since it would be easy to extend to arbitrary preconditions. Something to the effect of:

Create.Table("foo")
    .WithPrecondition(Preconditions.MustNotExist)
    .WithIdColumn();

Alter.Table("bar")
    .AddColumn("someCol")
    .Precondition(Preconditions.MustNotExist);

Delete.Table("foo")
    .WithPrecondition(Preconditions.MustExist);

Effectively, Preconditions are like our Conditionals today, except they do not actually connect to the database to determine their truthiness.

Advantages could be:

1. Possibly improve error logging. If a migration command fails, we can explicitly state any preconditions that should be checked.
2. It might be possible for FluentMigrator to maintain a MVCC-like data structure of object changes, such that it knows if the precondition is true or not. People could then even "inject" a Database "WORLD STATE" into the Runner, and do a dry run to assert truthiness.

[danpadmore]

I would love this feature, because currently I rely on preview for generating scripts, but Schema.Exists executes during script generation instead of generating the required script in the preview.