Major plugin refactor and cleanup.

Switched to POCO library for unified platform/library interface.
Deprecated the external module API. It was creating more problems than solving.
Removed most built-in libraries in favor of system libraries for easier maintenance.
Cleaned and secured code with help from static analyzers.

vendor/CPR/include/cpr/proxies.h

@@ -0,0 +1,22 @@
+#ifndef CPR_PROXIES_H
+#define CPR_PROXIES_H
+
+#include <initializer_list>
+#include <map>
+#include <string>
+
+namespace cpr {
+class Proxies {
+  public:
+    Proxies() = default;
+    Proxies(const std::initializer_list<std::pair<const std::string, std::string>>& hosts);
+
+    bool has(const std::string& protocol) const;
+    const std::string& operator[](const std::string& protocol);
+
+  protected:
+    std::map<std::string, std::string> hosts_;
+};
+} // namespace cpr
+
+#endif