feat(new): --with-ext to specify type without --file
feat(new): --with-ext to specify type without --file

With custom `new` files, the user can now have `liquid` defaults and not
just `md`.  The problem is you had to do:
`cobalt new "Page Title" --file page-title.liquid`
This now changes to
`cobalt new "Page Title" --with-ext liquid`

This also allows you to use multiple related extensions:
`cobalt new "Page Title" --file page-title.markdown --with-ext md`
This loads `pages.md` and saves it as `page-title.markdown`.

cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs