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

include/package_manager.hpp

@@ -27,6 +27,7 @@ namespace gdpm::package_manager {
 		update, 
 		search, 
 		p_export, /* reserved keyword */
+		purge,
 		list, 
 		link,
 		clone,