src: Support the -help flag

This commit allows a help message to be printed when a `-help` flag is
passed to the editor, which will subsequently quit after a summary and a
description of all the flags available have been displayed.

The GNU convention (passing a single `--help` argument to the program)
is also supported, although undocumented.

The man page also now documents the `+:` argument, although unrelated to
the original changeset.

doc/kak.1.txt

@@ -7,7 +7,7 @@ kak - a vim inspired, selection oriented code editor
 
 SYNOPSIS
 --------
-*kak* [-q] [-n] [-l] [-ro] [-clear] [-ui ui_type] [-e command] [-f keys] [-p session_id] [-c session_id|[[-d] -s session_id] [+line[:column]] file ...
+*kak* [-help] [-q] [-n] [-l] [-ro] [-clear] [-ui ui_type] [-e command] [-f keys] [-p session_id] [-c session_id|[[-d] -s session_id] [+line[:column]|+:] file ...
 
 DESCRIPTION
 -----------
@@ -26,6 +26,9 @@ stays fixed and the cursor one moves around.
 
 OPTIONS
 -------
+-help::
+	display a help message and quit
+
 -n::
 	do not load resource files on startup ('kakrc', 'autoload', 'rc' etc)
 
@@ -63,7 +66,8 @@ OPTIONS
 	enter in 'readonly mode', all the buffers opened will not be written to disk
 
 +line[:column]::
-	specify a target line and column for the first file
+	specify a target line and column for the first file; when the plus sign is followed by only a colon,
+	then the cursor is sent to the last line of the file
 
 file::
 	one or more files to edit

src/main.cc

@@ -801,12 +801,33 @@ int main(int argc, char* argv[])
                    { "ui", { true, "set the type of user interface to use (ncurses, dummy, or json)" } },
                    { "l", { false, "list existing sessions" } },
                    { "clear", { false, "clear dead sessions" } },
-                   { "ro", { false, "readonly mode" } } }
+                   { "ro", { false, "readonly mode" } },
+                   { "help", { false, "display a help message and quit" } } }
     };
+
     try
     {
+        auto show_usage = [&]()
+        {
+            write_stdout(format("Usage: {} [options] [file]... [+<line>[:<col>]|+:]\n\n"
+                    "Options:\n"
+                    "{}\n"
+                    "Prefixing a positional argument with a plus (`+`) sign will place the\n"
+                    "cursor at a given set of coordinates, or the end of the buffer if the plus\n"
+                    "sign is followed only by a colon (`:`)\n",
+                    argv[0], generate_switches_doc(param_desc.switches)));
+            return 0;
+        };
+
+        if (contains(ConstArrayView<char*>{argv+1, (size_t)argc-1}, StringView{"--help"}))
+            return show_usage();
+
         ParametersParser parser(params, param_desc);
 
+        const bool show_help_message = (bool)parser.get_switch("help");
+        if (show_help_message)
+            return show_usage();
+
         const bool list_sessions = (bool)parser.get_switch("l");
         const bool clear_sessions = (bool)parser.get_switch("clear");
         if (list_sessions or clear_sessions)