* The Nim binary now recognizes the following environment and allows them to
override configured values:
- `DATABASE_DRIVER`: overrides the `driver` config value, selects which
kind of database we expect to connect to.
- `MIGRATIONS_DIR`: overrides the `sqlDir` config value, sets the path to
the directory containing the migration SQL files.
- `DATABASE_URL`: overrides the `connectionString` config value, supplies
connection parameters to the database.
- `DBM_LOG_LEVEL`: overrides the `logLevel` config value, sets the logging
level of db_migrate.
* Fixes a bug in the parsing of migration files. Previously the file was split
on `;` characters and chunks that started with `--` were ignored as comments.
This was wrong in the common case where a block starts with a comment but
then contains SQL further. Such a block was being ignored. The new behavior
is to consider each line and build up queries that way.
* Fixes a logging bug when parsing the configuration. If there was an that
prevented the configuration from loading this in turn prevented logging from
being setup correctly, and so the error was not logged. Now errors that may
occur before logging is setup are hard-coded to be logged to STDERR.
* Changes the logic for creating the `migrations` table to check before
performing the query that creates the table. This is to avoid continually
printing messages about skipping this creation when the table already exists
(which is the normal case). This change is PostgreSQL-specific, but of course
the entire tool is currently PostgreSQL-specific.
