1004 execute migration as role

Add `set_role` method to migration.

## Changes
* Add `set_role` method to migration.
* Create `PgPower::Config`, with `ensure_role_set` settings. If it's true it forces all migrations to have set role.

## RSpec
```
Finished in 1.51 seconds (files took 1.22 seconds to load)
158 examples, 0 failures, 2 pending
```

## Doc
```
Files:          46
Modules:        42 (    0 undocumented)
Classes:        11 (    0 undocumented)
Constants:       8 (    0 undocumented)
Methods:       158 (    0 undocumented)
 100.00% documented
```