Add a prompt upon generating project to fetch dependencies

[mbramson]

Similar to how the phoenix phx.new generator works after generating the project scaffolding, it would be great if the user was prompted whether they wanted to fetch dependencies.

If a user responds with yes, the following commands should be executed:
mix deps.get
cd assets && yarn install (if the --webpack flag is used)

Hopefully this the clean_up functionality in mix templates can be utilized as documented here: https://github.com/pragdave/mix_templates/blob/master/lib/mix_templates.ex#L360