offline cache

[tripodsan]

Is your feature request related to a problem? Please describe.
it's not possible to develop offline - or with a slow connection - easily, eg when traveling in trains or planes.

Describe the solution you'd like
the cli should cache proxied content by default (with a reasonable limit) and detect offline conditions.
once detected, it should either transparently switch to the cached content, or offer to user to do so.

all this should also be configurable via cmd line arguments.