feat(new command): updates new to download multiple file templates

[philnash]

Uses the new layout of the functions-templates project to download multiple file templates into a
twilio-run project. Templates will be downloaded into the functions/{bundleName} and
assets/{bundleName} directories within the project.

This also allows for passing an empty bundleName (or filename in the twilio-run CLI parlance right
now). When empty, this adds the functions/assets directly into the functions/assets directories.
This will be used for create-twilio-function template options. The twilio-run CLI still enforces
a name.

BREAKING CHANGE: This needs the functions-templates repo to be up to date. Currently it points to
the next branch, which is up to date.

re #20

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[philnash]

Oh, speaking of the --filename argument, this name doesn't exactly make sense any more as there's a bundle of functions and assets. I've started calling it a bundle internally but wanted to get your opinion on that @dkundel. Is bundle a good name for the set of functions/assets that we download and is --bundle-name or a good name for the flag you can pass with --template when you start the command?

[dkundel]

How about namespace for it instead? Seems like the best description. It would als explain better if it doesn't exist