add RecursiveFileMigrationSource

[zaneli]

I’d like to move the excessive number of applied migration files while keeping the history table consistent.

Currently it looks like this:

migrations/1_aaa.sql
migrations/2_bbb.sql
migrations/3_ccc.sql
migrations/4_ddd.sql

I’d like it to look like this:

migrations/archive/1_aaa.sql
migrations/archive/2_bbb.sql
migrations/archive/3_ccc.sql
migrations/current/4_ddd.sql

As for the approach, I’m considering either (1) allowing multiple dirs to be specified, or (2) making it possible to process subdirectories recursively.

  dirs:
    - migrations/archive
    - migrations/current

  dir: migrations/*

In this PR, I’ve tried implementing approach (2).
If approach (1) is preferred, I’ll discard this PR and open a new one.