Caching libraries

**Describe the Feature**
Freezing has been failing for me this afternoon because
```
TypeError: Could not load https://jspm.dev/npm:immutable@4.0.0-rc.12!cjs: fetch failed
```
Twice it was due to a Connect Timeout Error, and once due to a SocketError: other side closed.

Presumably due to some outage at jspm.dev, or jspm.dev got too budy, or...I will try again later.
But it did make me wonder why the freezer doesn't download the node_modules (as node applications do) and use the cached version of the library rather than fetching on every build.

**Graphics**
If applicable, provide screenshots/videos of similar ideas in other software.

**Implementation Details**
If applicable, provide insights with regards to implementing this feature that you already have.

Start by tracing the freezer's current fetch of libraries from jspm.dev and how dependencies are handled during builds. The work is done when the freezer can reuse a locally cached node_modules library on later builds instead of fetching it every time, including when jspm.dev is unavailable.

Written by the indexing model from the issue text.