Add option to bin/importmap pin URL to create direct/remote link without downloading

[bensheldon]

It's currently possible to "manually" add a URL to config/importmap.rb that will not attempt to be locally cached e.g.

# config/importmap.rb
pin "some-package", to: "https://someotherhost.com/some-package.js"

...but there isn't currently a way to add that entry through the $ bin/importmap pin command because it will always attempt to download and vendor the file through jspm:

importmap-rails/lib/importmap/commands.rb

Lines 15 to 19 in ddf9be4

	def pin(*packages)
	if imports = packager.import(*packages, env: options[:env], from: options[:from])
	imports.each do |package, url|
	puts %(Pinning "#{package}" to #{packager.vendor_path}/#{package}.js via download from #{url})
	packager.download(package, url)

This Issue proposes an option for adding a new entry using bin/importmap pin that will not be downloaded and maintain the direct URL:

bin/importmap pin https://mycdn.com/some-package.js --from direct

Update: Thinking some more about this, I wonder if when using the CLI pin command, merely passing a value that starts with http[s]:// would result in the direct mode.

Update2: Oops, I overlooked the change where pin is now synonymous with vendoring the library. I think maybe I have two thoughts now on this:

1. The behavior of providing a static URL does still seem to work when added to the config/importmap.rb file.
2. The use-case behind this is a desire to use my own js libraries that are served from their own remote host and are available upon multiple websites/projects and I'd like to update/manage them remotely without vendoring/updating/managing them within multiple individual Rails apps. Unfortunately browser imports only allow a single <script type="importmap"> ... tag per HTML page, which means that all importmap definitions would need to go through a single helper.