Backward-compatible sqlite autoincrement

Hello, 

Here is another attempt to introduce sqlite monotonic autoincrement which tries to be back-compatible.

Without extending ```SQL::Translator::Schema::Field``` I decided to use ```extra``` field to store type of autoincrement, which later could be re-used from Producer.

I hope this would be OK.

I tried to use it directly in ```DBIx::Class``` :

```perl

{ data_type => "integer", is_auto_increment => 1, is_nullable => 0, extra => { autoinc_method => 'sequence' } }

```

- it works that way. But how to specify extra more correctly in DBIx::Class? i..e outside of auto-generated sources?

Thanks for the great project!