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/http/README.md

@@ -1,15 +1,71 @@
-# GDPM HTTP Example
+# GDPM Rest API Example
 
-This is an example showing how to use the GDPM HTTP library to download files. The library uses RapidJSON to get results.
+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.
 
+Here is a snippet making a HTTP get and post request.
 
-Here is a quick snippet to get started:
 ```c++
-// 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
+#include "http.hpp"
+
+#include <string>
+#include <unordered_map>
+
+using string 	= std::string;
+using headers_t = std::unordered_map<string, string>;
+
+string url = "www.example.com";
+http::context http;
+http::request params = {{"user-agent", "firefox"}};
+http::response r = http::request(url, params);
+if(r.code == http::OK){ /* ...do something... */ }
+
+r = http::request(url, params, http::method::POST);
+if(r.code == http::OK){ /* ...do something.. */ }
+```
+
+Here's an example using the `multi` interface for parallel requests and downloads:
+
+```c++
+#include "http.hpp"
+
+#include <memory>
+
+template <typename T>
+using ptr 		= std::unique_ptr<T>;
+using strings 	= std::vector<std::string>;
+using transfers = std::vector<http::transfer>;
+using responses = std::vector<http::response>;
+
+http::multi http;
+http::request params = {{"user-agent", "firefox"}}
+params.headers.insert(http::header("Accept", "*/*"));
+params.headers.insert(http::header("Accept-Encoding", "application/gzip"));
+params.headers.insert(http::header("Content-Encoding", "application/gzip"));
+params.headers.insert(http::header("Connection", "keep-alive"));
+
+strings request_urls = {
+	"www.example.com",
+	"www.google.com"
+}
+ptr<http::transfers> requests = http::make_requests(request_url, params);
+ptr<http::responses> responses = http::execute(std::move(requests));
+for(const auto& r : *responses){
+	if(r.code == http::OK){
+		// ...do something
+	}
+}
+
+strings download_urls = {
+	""
+}
+strings storage_paths = {
+	"./download1"
+}
+ptr<http::transfers> downloads = http::make_downloads(download_url, storage_paths, params);
+ptr<http::responses> responses = http::execute(std::move(downloads));
+for(const auto& r : *responses){
+	if(r.code == http::OK){
+		// ...do something
+	}
+}
 ```