Support multiple --migrations-dir directories, merging their contents

[dossy]

It would be great if I could specify multiple --migrations-dir (-d) arguments in an execution, and have dbmate read the migrations files from all of the directories and interleave (merge + sort) the files together, and apply them in the newly sorted order.

There are two very distinct use cases I want this functionality for:

1. Schema migrations separate from seed (test) data

In the simplest scenario, I want to keep schema migrations (DDL) in db/migrations, and only apply those into my production environment. However, for my dev/test environments, I want to load seed (test) data.

I could duplicate everything in db/migrations into a db/seeds, and then have my seed data files mixed in chronologically where they belong. This duplication of file artifacts isn't ideal, but it "solves" the problem.

A better solution would be only keeping the common migrations in db/migrations and only the seed files in db/seeds, and then running dbmate with --migrations-dir db/migrations against production, and --migrations-dir db/migrations --migrations-dir db/seeds in dev/test, and have dbmate merge the contents of the two directories together before applying them.

Note: This isn't just concatenating the list of files in the second specified directory onto the end of the list of files in the first specified directory. This is interleaving them together based on the filenames, sorting them into one single list.

2. Common migrations across environments, with environment-specific additions

Similar to the first scenario above, perhaps I have a shared set of migrations which I want to store in db/migrations, but then have special migrations for dev in db/migrations/dev and for testing in db/migrations/test, etc. Being able to specify multiple --migrations-dir locations and having dbmate merge and the contents and applying them.

Note: This isn't just about being able to specify only two --migrations-dir locations, but two or more, and having them all merged and sorted together.

---

This is functionality I need, personally, so I plan on working on implementing this, but I'd like to know if anyone else is similarly interested in such a feature, and discuss any potential issues or gotchas with the feature as I've envisioned it.

[dossy]

For anyone who is interested in this, and wants to help test:

#428

[gregwebs]

This feature is merged now, great work!