Add a --verbose option to the mvt, mvt-ios and mvt-android commands

Verbosity was an option of each module-running command, so
"mvt-ios --verbose check-backup" was a usage error, a plugin command had to
define a flag of its own, and there was no way to get debug output from mvt at
all.

The option now sits on the three commands themselves and sets the level of
MVT's console handler for the run, through set_verbose_logging(), before any
command runs. Plugin commands registered on any of the three CLIs get the
option for free and need none of their own.

The per-command --verbose of the check-* commands is kept for backward
compatibility. It only ever raises the level, so the CLI's choice is never
undone by a command's default, and its help text says it is kept for
compatibility. It is to be removed in a later release.

README.md

@@ -60,6 +60,8 @@ For alternative installation options and known issues, please refer to the [docu
 
 MVT provides three commands: `mvt-ios` and `mvt-android` analyse acquisitions from devices of that platform, and `mvt` hosts what belongs to neither: `version`, `completion`, `plugins` and `download-iocs` (`version` and `download-iocs` remain available on the platform commands for now). Running `mvt` on its own shows the installed version, update notices and the available commands. [Check out the documentation to learn how to use them!](https://docs.mvt.re/)
 
+Pass `--verbose` to any of the three commands, before the command name (`mvt-ios --verbose check-backup ...`), for debug output. The `--verbose` option the `check-*` commands accept after their name still works but is kept for compatibility only and will be removed in a future release.
+
 ### Shell completion
 
 MVT can generate a shell completion script for Bash, Zsh, and Fish which covers `mvt`, `mvt-ios` and `mvt-android`: