add migration table name as optional parameter

Sometimes it's useful/needed to be able to control the migrations table name.

The pull request is adding a :table option  to the ragtime configuration map that will override the default name("ragtime_migrations"). If no table option is passed the migration table will still use the regular *ragtime_migrations*

```clojure

{:migrations ragtime.sql.files/migrations
 :database "jdbc:mysql://localhost:3306/example_db?user=root"
  :table "table_name_i_like"}
````
Will initially create and then use a table *called table_name_i_like* to keep track of the applied migrations.