Add the short option for help command: -h

It's much easier for people use `-h` command-line switch instead of longer `--help` to
retrieve the help about CLI or a specific command.  This PR extends the current behavior
by allowing the short option as well in addition to longer one.

sigma/cli/main.py

@@ -37,7 +37,12 @@
 from .pysigma import check_pysigma_command
 
 
-@click.group()
+CONTEXT_SETTINGS={
+    "help_option_names": ['-h', '--help']
+}
+
+
+@click.group(context_settings=CONTEXT_SETTINGS)
 def cli():
     pass