Allow ignoring initial migrations

Sometimes, if your apps have cyclic dependencies or complicated FK structure,
django migrations creates multiple initial migration files, which can very
easily fail linting because they are adding foreign keys or constraints etc.

This adds an option to automatically ignore all initial migrations.

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 5.x.y
 
+Feature:
+- Allow ignoring all initial migrations, with `--ignore-initial-migrations`
+
 Bug:
 - Don't detect an index creation during a transaction with an exclusive lock, when the table is being created (#264)