"read from url" should cache http content

[joaompinto]

Retrieved files should be cached depending on:
https://developer.mozilla.org/pt-BR/docs/Web/HTTP/Headers/Last-Modified
https://developer.mozilla.org/pt-BR/docs/Web/HTTP/Headers/ETag

It should be possible to configure the caching behavior by introducing a cache configure option:
cache: auto # Can be set to "never"