Implement the ability to reference URLs in bundles.

[farmdawgnation]

While tossing around ideas about how to improve pulling in external libraries when using sbt-resource-management, we came up with the idea of referencing libraries by URL in the bundle file. Such as:

base
http://code.jquery.com/jquery-2.1.3.min.js
my-thing.js

When copying scripts, sbt-resource-management would fetch jquery-2.1.3.min.js and stick it in the target JS folder which is, in theory, outside of source control. This prevents the need to manually download an external script into source control in your project.

[Shadowfiend]

Thinking the mechanic for this should be:

• Warn on non-https URI.
• If the file already exists locally, don't redownload it (no modification checking or anything).