Add start_index argument to 'parse' function

Usual case for emulation programs, like sde or gdb, is to pass first
arguments to the emulator, and let the guest application handle latter
arguments. To support this behavior, we introduce an option to choose
index of the first argument to be handled by POPL. By default it is 1,
as it is usually corresponds to the binary name.

include/popl.hpp

@@ -326,7 +326,8 @@ class OptionParser
 	/// Parse the command line into the added Options
 	/// @param argc command line argument count
 	/// @param argv command line arguments
-	void parse(int argc, const char * const argv[]);
+	/// @param start_index index of starting argument
+	void parse(int argc, const char * const argv[], int start_index = 1);
 
 	/// Produce a help message
 	/// @param max_attribute show options up to this level (optional, advanced, expert)
@@ -916,14 +917,14 @@ inline std::shared_ptr<T> OptionParser::get_option(char short_name) const
 }
 
 
-inline void OptionParser::parse(int argc, const char * const argv[])
+inline void OptionParser::parse(int argc, const char * const argv[], int start_index)
 {
 	unknown_options_.clear();
 	non_option_args_.clear();
  	for (auto& opt : options_)
 		opt->clear();
 
-	for (int n=1; n<argc; ++n)
+	for (int n=start_index; n<argc; ++n)
 	{
 		const std::string arg(argv[n]);
 		if (arg == "--")