Set migration table using an environment variable.

Example usage:

MIGRATION_TABLE=bunnies sql-migrate up

Or:

export MIGRATION_TABLE=bunnies 
sql-migrate up


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Notes: This is for discussion purposes. I haven't yet added any documentation. It was more difficult than expected to add a command-line flag without adding duplicate code to several files, so I avoided that for now.

If this approach is accepted then I'll add documentation before expecting this pull request to be accepted. In the meantime, it's a fix for @ciarand which does not break compatibility.