add pre_hook/post_hook aliases to config (#1124)

[beckjake]

Fixes #1124 by allowing users to use pre_hook and post_hook in keyword arguments to the config. If a user puts both pre-hook and pre_hook into a config's arguments, dbt will raise a compiler error:

Encountered an error:
Compilation Error in model data (models/data.sql)
  Invalid config, has conflicting keys "pre_hook" and "pre-hook"

[drewbanin]

Looks good to me!