Add execute_file/1 and /2

[greg-rychlewski]

Adds a small helper for people storing their execute migrations inside of files. Instead of

execute File.read!(file)
execute File.read!(file1), File.read!(file2)

it is

execute_file file
execute_file file1, file2

[josevalim]

PR looks great to me but I wonder if we need those to be integration tests. I think a unit test is fine, especially beacuse it builds on top of the already tested execute functionality?

[greg-rychlewski]

Makes sense. Thank you!