Implemented parallel downloads through CURL multi interface and added purge command

- Update `README.md` file with examples
- Fixed error messages showing no or wrong message
- Changed the prompt message when installing, removing, etc.
- Changed how http::request works
- Added `http::multi` class for parallel downloads
- Removed separate `concepts.hpp` file
TODO: Fix ZIP not extracting after running the `install` command

examples/rest_api/README.md

@@ -1,26 +1,15 @@
-# GDPM Rest API Example
+# GDPM HTTP Example
 
-This is an example showing how to use the REST API designed to query the Godot Asset library in C++. It is built using the `libcurl` library.
+This is an example showing how to use the GDPM HTTP library to download files. The library uses RapidJSON to get results.
 
-Here is a snippet making a HTTP get and post request.
 
+Here is a quick snippet to get started:
 ```c++
-using string 	= std::string;
-using headers_t = std::unordered_map<string, string>;
-
-std::string url = "www.example.com";
-http::response r_get = http::request_get(url)
-if(r_get.response_code == http::response_code::OK){
-	// ...do something...
-}
-
-http::request_params params;
-params.headers = {
-	{"user-agent", "firefox"},
-	{"content-type", "application/json"}
-}
-http::response r_post = http::request_post(url, params);
-if(r_post.response_code == http::response_code::OK){
-	// ...do something...
-}
+// Get a full list of assets
+rest_api::context context = rest_api::make_context();
+rapidjson::Document doc = http::get_asset_list(
+	"https://godotengine.org/asset-library/api",
+	context
+)
+// ... parse the rapidjson::Document
 ```