Migrations: Write script to .sql file (Fixes #420)

This is more-or-less the same logic we used in EF6.
Wiil this work on non-English VS? http://social.msdn.microsoft.com/Forums/vstudio/en-US/eb032063-eb4d-42e0-84e8-dec64bf42abf/how-can-i-create-items-in-visual-studio-in-a-localeindependent-way?forum=vsx