[Research and implementation] Figure out if the 'urfave/cli' library can be shrunk, if not, use smaller cli library

Currently the Mender-client is running with the 'urfave/cli' library, which we are quite happy with, except for its humongous size!

The library is weighing in at 30% of the total client size, which is completely off the mark.

There were a few original suggestions to replace it with something smaller:

• https://tracker.mender.io/browse/MEN-3058
• Which, if we change, leads to https://tracker.mender.io/browse/MEN-3059

Therefore, it should probably be possible to shrink the cli-library through any number of approaches, like conditional compilation, or simple removing the parts we don't want in a fork.

Acceptance criteria:

• Figure out if the 'urfave/cli' library can be shrunk to an acceptable size
• Implement the chosen solution on the client.