Override options from command line

Allow any configuration option to be overridden with -o/--override
'section.key=value' arguments, as suggested in #554

update completitions for override

slight refactoring to ease footclient support

completions/zsh/_foot

@@ -4,6 +4,7 @@ _arguments \
     -s -S -C \
     '(-c --config)'{-c,--config}'[path to configuration file (XDG_CONFIG_HOME/foot/foot.ini)]:config:_files' \
     '(-C --check-config)'{-C,--check-config}'[verify configuration and exit with 0 if ok, otherwise exit with 1]' \
+    '(-o --override)'{-o,--override}'[configuration option to override, in form SECTION.KEY=VALUE]:()' \
     '(-f --font)'{-f,--font}'[font name and style in fontconfig format (monospace)]:font:->fonts' \
     '(-t --term)'{-t,--term}'[value to set the environment variable TERM to (foot)]:term:->terms' \
     '(-T --title)'{-T,--title}'[initial window title]:()' \