Add add_foreign to the schema builder

[josephmancuso]

Describe the feature as you'd like to see it
We should have an add_foreign method on the schema builder. This should be a shortcut to the current way to add foreign keys. It should look like this:

table.add_foreign("from_column.to_column.table")

The above would be the same as this:

table.foreign("from_column").references("to_column").on("table")