Add a platform-neutral mvt command

Several MVT commands have nothing to do with the acquisition of one
platform, yet they were reachable only through mvt-ios and mvt-android.
Asking which version is installed or downloading the public indicators
meant picking one of the two platform commands arbitrarily, and each of
those tasks had to be written, documented and maintained twice.

Add a third console script, mvt, hosting the commands which belong to no
platform: version and download-iocs for now, with completion following
in a later commit.

Commands installed in the new mvt.cli_plugins entry-point group are
registered on mvt, and on mvt only, so that a command package chooses
the CLI each of its commands is added to: mvt.ios.cli_plugins for
mvt-ios, mvt.android.cli_plugins for mvt-android and mvt.cli_plugins for
mvt. A command wanted on both platform CLIs is registered in both
platform groups; no group adds a command to every CLI. The
MVT_CUSTOM_COMMANDS variable loads command files and folders into mvt
the way the platform variables already do for mvt-ios and mvt-android.

Run on its own, mvt prints the banner and its help instead of a usage
error. The help text reminds that the forensic analysis of an
acquisition runs through mvt-ios and mvt-android, so that the command
which knows nothing about acquisitions says where they are analysed.

version and download-iocs stay on mvt-ios and mvt-android for now, so
that no documented invocation stops working. They are to be dropped from
the platform CLIs in a later release, once mvt has been available long
enough for the change to be announced.

Unlike mvt-ios and mvt-android, which point at their subpackages, the
console script points at mvt.cli:main and the mvt package re-exports
nothing of it. Importing mvt has to stay cheap and free of side effects:
it is the package plugins import from, and pulling in Click, the CLI and
everything the commands import merely because something imported mvt
would work against that.

While here, give the version command of both platform CLIs the context
settings every other command already has, so that "mvt-ios version -h"
prints its help instead of failing on an unknown option.

README.md

@@ -58,7 +58,7 @@ For alternative installation options and known issues, please refer to the [docu
 
 ## Usage
 
-MVT provides two commands `mvt-ios` and `mvt-android`. [Check out the documentation to learn how to use them!](https://docs.mvt.re/)
+MVT provides three commands: `mvt-ios` and `mvt-android` analyse acquisitions from devices of that platform, and `mvt` hosts what belongs to neither: `version` and `download-iocs` (both 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/)
 
 ### Shell completion
 
@@ -82,8 +82,8 @@ Module-running `check-*` commands can load custom Python modules with
 [development documentation](https://docs.mvt.re/en/latest/development/) for
 details.
 
-Users can also add top-level commands to `mvt-ios` and `mvt-android` from
-installed Python packages or local files and folders. See the
+Users can also add top-level commands to `mvt`, `mvt-ios` and `mvt-android`
+from installed Python packages or local files and folders. See the
 [custom CLI command documentation](https://docs.mvt.re/en/latest/development/custom_commands/)
 for the plugin entry points and `--load-command` interface.