New feature: Offline mode

[screamerbg]

Now that the mbed cache is a default Mbed CLI feature (since v1.5.0), a new offline mode feature can take benefit from it. Offline mode is achieved by utilize the cache for various Mbed CLI operations and also preventing Mbed CLI to access remote repositories to avoid connection errors while in offline.

Workflow
No impact to existing workflows.

This feature adds --offline switch to mbed import, mbed add, mbed update and mbed new to enable offline mode as described above.

How this works
While using --offline for mbed import/add/update/new, Mbed CLI will look up for locally cached repositories and use them without fetching new content from remote repositories.

• If a repository is not cached, an error will be printed that the repository cannot be imported due to offline mode being enabled (even if connection is present).
• If a repository is cached, there might be a corner case where one cached repository references another cached repository, but the local cache for the later is older and therefore the referenced hash is missing.

Lastly, Mbed CLI will print a banner when --offline is used to inform the user that they are in offline mode.

$ mbed import https://github.com/ARMmbed/mbed-os-example-mesh-minimal --offline

.=============================== OFFLINE MODE ================================.
|   Offline mode is enabled. No connections to remote repositories will be    |
|           made and only locally cached repositories will be used.           |
|   This might break some actions if non-cached repositories are referenced.  |
'============================================================================='

[mbed] Importing program "mesh-minimal" from "https://github.com/ARMmbed/mbed-os-example-mesh-minimal" at latest revision in the current branch
[mbed] Adding library "atmel-rf-driver" from "https://github.com/ARMmbed/atmel-rf-driver" at rev #2cd9abba3e37
[mbed] Adding library "mbed-os" from "https://github.com/ARMmbed/mbed-os" at rev #ad284b28064b
[mbed] Adding library "mcr20a-rf-driver" from "https://github.com/ARMmbed/mcr20a-rf-driver" at rev #9aac10474702
[mbed] Adding library "sd-driver" from "https://github.com/ARMmbed/sd-driver" at rev #51dc1d2b2c07

Documentation
Documentation is not included with this PR. @AnotherButler please advise where offline mode should be mentioned. Perhaps in https://github.com/ARMmbed/mbed-cli#repository-caching or https://github.com/ARMmbed/mbed-cli#importing-an-existing-program, or both?

Tests
mbed new --offline and mbed update --offline is tested via circle.yml

Resolves #351

[theotherjimmy]

Looks fine to me.

[theotherjimmy]

Testing might be improved by forcing the network to not be accessible for the part of the tests that test this feature.

[sg-]

👍

[AnotherButler]

I would think repository caching would be a good place.

[screamerbg]

Just rebased this on top of master (1.5.1) and added documentation entry for offline mode. @AnotherButler please review and make changes where necessary.

[screamerbg]

Bump @AnotherButler