harmonize header inclusion style

close #7
I don't intend this patch as a piece of art, but it carries the main idea

src/main.cc

@@ -1,9 +1,10 @@
-#include <iostream>
-#include <fstream>
 #include "actions.h"
 #include "options.h"
 #include "version.h"
 
+#include <iostream>
+#include <fstream>
+
 static void show_usage(const bool include_help)
 {
     static const auto usage =

src/options.cc

@@ -1,6 +1,5 @@
-#include <getopt.h>
-#include <regex>
-#include <sstream>
+#include "options.h"
+
 #include "tags_handlers/insertion_tags_handler.h"
 #include "tags_handlers/modification_tags_handler.h"
 #include "tags_handlers/external_edit_tags_handler.h"
@@ -8,7 +7,10 @@
 #include "tags_handlers/import_tags_handler.h"
 #include "tags_handlers/listing_tags_handler.h"
 #include "tags_handlers/removal_tags_handler.h"
-#include "options.h"
+
+#include <getopt.h>
+#include <regex>
+#include <sstream>
 
 using namespace opustags;
 

src/options.h

@@ -1,9 +1,10 @@
 #pragma once
 
+#include "tags_handlers/composite_tags_handler.h"
+
 #include <stdexcept>
 #include <map>
 #include <vector>
-#include "tags_handlers/composite_tags_handler.h"
 
 namespace opustags
 {

tests/actions_test.cc

@@ -1,13 +1,13 @@
-#include "ogg.h"
 #include "actions.h"
 #include "catch.h"
 
-#include <fstream>
-#include <cstring>
-
+#include "ogg.h"
 #include "tags_handlers/insertion_tags_handler.h"
 #include "tags_handlers/stream_tags_handler.h"
 
+#include <fstream>
+#include <cstring>
+
 using namespace opustags;
 
 static bool same_streams(ogg::Decoder &a, ogg::Decoder &b)

tests/options_test.cc

@@ -1,6 +1,6 @@
 #include "options.h"
-#include <memory>
 #include "catch.h"
+
 #include "tags_handlers/export_tags_handler.h"
 #include "tags_handlers/external_edit_tags_handler.h"
 #include "tags_handlers/import_tags_handler.h"
@@ -9,6 +9,8 @@
 #include "tags_handlers/modification_tags_handler.h"
 #include "tags_handlers/removal_tags_handler.h"
 
+#include <memory>
+
 using namespace opustags;
 
 static std::unique_ptr<char[]> string_to_uptr(const std::string &str)