Allow custom migration template

[lewismoten]

I would like an option to provide a custom template for migrations & seeds so that the resulting output would look like the following:

const sql = require('../sql');

module.exports = {
    "up": sql``,
    "down": sql``
}

I use a VS Code plugin ( vscode-sql-template-literal ) that allows syntax highlighting of SQL within strings.

https://github.com/kawadhiya21/mysql-migrations/blob/65c52086c766d5c91508ba66c3b811682484c4b2/core_functions.js#L15