Allow boolean flags to be repeated

bat prepends the arguments from its config file to the command line, so a
user who has `--show-all` in their config and also types `-A` passes the
flag twice, and clap rejects that with "the argument '--show-all' cannot be
used multiple times" and exit code 2. The file is never displayed.

`--plain`, `--number`, `--number-nonblank`, `--force-colorization` and
`--no-paging` already avoid this by overriding themselves. The same is now
done for the eight remaining flags that affect how a file is displayed:
--show-all, --diff, --chop-long-lines, --squeeze-blank, --unbuffered,
--quiet-empty, --no-custom-assets and --set-terminal-title.

Repeating a flag produces byte-identical output to giving it once, and
--show-all still conflicts with --language as before.

tests/examples/bat-show-all.conf

@@ -0,0 +1 @@
+--show-all