Add TypeScript support

Since coffeescript is supported, i think it would be cool to have a typescript support also.

Expectation: run node_modules/sequelize-cli/bin/sequelize model:create --name Collection --attributes id:integer,name:string --typescript

Will produce a model which ends in '.ts' instead of '.js'.

Justification: In projects where typescript is used, javascript files are sometimes ignored (because they are produced through typescript). Therefore, the model will be ignored by default. Afterwards, a developer must edit the model/migration.

Result: Tiny bit of work removed when developing with typescript.