Add an option to disable checking for updates

[shubheksha]

Is this a bug or feature request?

Feature request

What is the current behavior?

In #676, we added support for allowing web-ext to alert the user whenever their version of web-ext becomes outdated.

What is the expected or desired behavior?

We should add support for a --disable-update-check option so that the user can turn this feature off as needed.

Additionally, reduce the time interval for checking updates from 7 days to 2-3 days. See #676.

[SomeoneSerge]

Relevant for offline (sandboxed) builds

[poly2it]

Any progress on this? This warning propagates in my sandboxed build containers.