single header include and compile flag for http

the example http_client.cpp shows to include the headers in following
order:

```
```

However, a common setting in auto-formatters like clang-format will sort
the headers in alphabetical order (which is also a requirement in some
code-styles), like this:

```
```

But this will lead to compilation errors as `netlib.hpp` includes
headers that are needed by `client.hpp`, for example  `<future>` but
also library-defined symbols like `DEFAULT_TIMEOUT` or `TICK_TIME`,
which are then included or defined after `client.hpp` needs them.

The ordering of includes of all needed library-headers should not
matter, but a single header file to include everything would be
preferable anyway. To exclude features like HTTP by default anyway, a
compile switch is added.

Author: maj0ran

examples/http_client.cpp

@@ -1,6 +1,6 @@
+#define NETLIB_USE_HTTP
+
 #include "../src/netlib.hpp"
-#include "../src/http/client.hpp"
-#include "../src/http/http.hpp"
 #include <csignal>
 #include <iomanip>
 #include <iostream>

src/netlib.hpp

@@ -2,4 +2,8 @@
 
 #include "client.hpp"
 #include "server.hpp"
-#include "thread_pool.hpp"
+#include "thread_pool.hpp"
+
+#ifdef NETLIB_USE_HTTP
+#include "http/client.hpp"
+#endif