Added MessageBox program

Added the program.

Author: ChrisBlueStone

Makefile

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+messagebox.exe: main.cpp
+	$(CXX) -o messagebox.exe main.cpp -O2 -s -mwindows

README.md

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 # messagebox
-Simply calls MessageBox on Windows and returns the result.
+Simply calls MessageBox using the Windows API and returns the result.
+
+This is very useful if you want to set reminders through Task Scheduler, so much so that I don't understand why this isn't a standard app.
+
+## Example usage
+The syntax is "messagebox *content* *title* *type*" where `type` is an integer representing various properties of the message box. The full list of message box types and return codes are available in the Microsoft documentation for MessageBox [here](https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-messagebox).
+
+```
+messagebox "This is the content of the message" "Title" 0
+```
+
+Since the result of the messagebox is returned, its value is stored in the `errorlevel` variable.
+```bat
+start /wait messagebox Continue? Confirm 4
+echo %errorlevel%
+```

main.cpp

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+#include <winuser.h>
+#include <cstdlib>
+
+int main(int argc, char** argv) {
+    return MessageBoxA(
+        NULL,
+        argc > 1 ? argv[1] : "",
+        argc > 2 ? argv[2] : "",
+        argc > 3 ? atoi(argv[3]) : 0
+    );
+}