Move get_cfg and usage out of compton.c

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/argparse.h

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
+#pragma once
+
+#include <stdbool.h>
+
+#include "compiler.h"
+
+typedef struct session session_t;
+
+/**
+ * Process arguments and configuration files.
+ */
+void get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass);