Add dropColumns method to Blueprint.php

[intrepidws]

When needing to drop multiple columns within a migration, I always find myself writing dropColumns() rather than dropColumn(). This is a minor paper cut but something that happens often enough that I felt it was worth submitting a PR to add this alias method to the schema builder.

[taylorotwell]

This already exists as Schema::dropColumns()

[intrepidws]

This already exists as Schema::dropColumns()

I saw that, but the usage for Schema::dropColumns() is quite different from the table-based syntax of dropColumn. That's where I constantly run into the issue of trying to use the plural dropColumns.