Run Argumnet::action functor for zero-parameter arguments

Previously, only arguments with one or more parameters would run actions.
But, at times it can be useful to run an action when an argument does not
expect any parameters.

Closes #104

Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

README.md

@@ -200,6 +200,24 @@ auto colors = program.get<std::vector<std::string>>("--color");  // {"red", "gre
 
 Notice that ```.default_value``` is given an explicit template parameter to match the type you want to ```.get```.
 
+#### Repeating an argument to increase a value
+
+A common pattern is to repeat an argument to indicate a greater value.
+
+```cpp
+int verbosity = 0;
+program.add_argument("-V", "--verbose")
+  .action([&](const auto &) { ++verbosity; })
+  .append()
+  .default_value(false)
+  .implicit_value(true)
+  .nargs(0);
+
+program.parse_args(argc, argv);    // Example: ./main -VVVV
+
+std::cout << "verbose level: " << verbosity << std::endl;    // verbose level: 4
+```
+
 ### Negative Numbers
 
 Optional arguments start with ```-```. Can ```argparse``` handle negative numbers? The answer is yes!

include/argparse/argparse.hpp

@@ -442,6 +442,7 @@ public:
     mUsedName = usedName;
     if (mNumArgs == 0) {
       mValues.emplace_back(mImplicitValue);
+      std::visit([](auto &aAction) { aAction({}); }, mAction);
       return start;
     } else if (mNumArgs <= std::distance(start, end)) {
       if (auto expected = maybe_nargs()) {

test/test_actions.cpp

@@ -133,3 +133,27 @@ TEST_CASE("Users can use actions on remaining arguments" *
   program.parse_args({"sum", "42", "100", "-3", "-20"});
   REQUIRE(result == 119);
 }
+
+TEST_CASE("Users can run actions on parameterless optional arguments" *
+          test_suite("actions")) {
+  argparse::ArgumentParser program("test");
+
+  GIVEN("a flag argument with a counting action") {
+    int count = 0;
+    program.add_argument("-V", "--verbose")
+      .action([&](const auto &) { ++count; })
+      .append()
+      .default_value(false)
+      .implicit_value(true)
+      .nargs(0);
+
+    WHEN("the flag is repeated") {
+      program.parse_args({"test", "-VVVV"});
+
+      THEN("the count increments once per use") {
+        REQUIRE(program.get<bool>("-V"));
+        REQUIRE(count == 4);
+      }
+    }
+  }
+}