mu: implement new command-line parser

Implement a new command-line parser, based on CLI11.

It's a bit more C++'ish, and allows for a lot of fancy things... some of
which we have implemented here.

Update the various commands to use the new Options struct

Remove the old help strings; instead e.g. `mu help view` opens the
manpage.

Integrate the guile scripts more tightly.

mu/meson.build

@@ -1,4 +1,4 @@
-## Copyright (C) 2021 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+## Copyright (C) 2021-2022 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 ##
 ## This program is free software; you can redistribute it and/or modify
 ## it under the terms of the GNU General Public License as published by
@@ -14,15 +14,10 @@
 ## along with this program; if not, write to the Free Software Foundation,
 ## Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 
-awk_script=join_paths(meson.current_source_dir(), 'mu-help-strings.awk')
-mu_help_strings_h=custom_target('mu_help',
-				input: 'mu-help-strings.txt',
-				output: 'mu-help-strings.inc',
-				command: [awk, '-f', awk_script, '@INPUT@'],
-				capture: true)
 mu = executable(
   'mu', [
   'mu.cc',
+  'mu-options.cc',
   'mu-cmd-cfind.cc',
   'mu-cmd-extract.cc',
   'mu-cmd-fields.cc',
@@ -30,11 +25,7 @@ mu = executable(
   'mu-cmd-index.cc',
   'mu-cmd-script.cc',
   'mu-cmd-server.cc',
-  'mu-cmd.cc',
-  'mu-cmd.hh',
-  'mu-config.cc',
-  'mu-config.hh',
-  mu_help_strings_h
+  'mu-cmd.cc'
 ],
   dependencies: [ glib_dep, gmime_dep, lib_mu_dep, thread_dep, config_h_dep ],
   cpp_args: ['-DMU_SCRIPTS_DIR="'+ join_paths(datadir, 'mu', 'scripts') + '"'],