(fixes #744) deprecate sql_where and provide an alternative

[drewbanin]

Example usage:

{{ config(materialized='incremental') }}

select * from public.events

{% if is_incremental() %}
    where dvce_created_tstamp > (select max(dvce_created_tstamp) from {{ this }})
{% endif %}

TODO:

• add a deprecation warning when sql_where is provided
• make sql_where no longer required
• make it easier/faster to do the "advanced" version of incremental models
• Add tests for is_incremental() usage
• revamp docs around incremental models (Update incremental model docs to explain how and when to use sql_where (never) #1036)
• Change link to docs in the PR (to something hidden for now)

New incremental docs are here: https://docs.getdbt.com/v0.12/docs/configuring-incremental-models

[beckjake]

lgtm