Add templates for asset-functions that serve Javascript and CSS

[parruda]

The idea is to create a template for the command faastruby new that would initialize a folder with a function already prepared to serve Javascript and CSS.
The command could be, for example:

~$ faastruby new FUNCTION_NAME --template assets

Note that we must have templates for all the supported runtimes (--runtime).

The resulting folder could look like this:

function_folder
├── assets
│   ├── javascript
│   │   └── application.js
│   └── stylesheets
│       └── application.css
├── spec
│   ├── handler_spec.rb
│   └── spec_helper.rb
├── Gemfile
└── handler.rb

Example requests:

• /workspace-name/function-name?css=application.css
• /workspace-name/function-name?js=application.js

And an example Ruby function could be like:

def handler(event)
  return render(css: File.read("assets/stylesheets/#{event.query_params['css']}")) if event.query_params.has_key?('css')
  return render(js: File.read("assets/javascript/#{event.query_params['js']}")) if event.query_params.has_key?('js')
end

There's also the need for sanitizing the input. For example, we don't want to allow js=../foo.js (which would not result in anything bad because the platform is secured against stuff like that - but it is better to avoid anyways). So there's a need for some sort of regular expression to remove everything fromcss= and js= but letters, numbers, dashes, underlines and one period.
The above is just a suggestion. Feel free to propose something better.

[parruda]

It would also be great if this could create a function from a template that is hosted in a remote Git repository. For example:

~$ faastruby new FUNCTION_NAME --template github:parruda/faastruby-assets-template

[parruda]

Closed in favor of #8