Make input files required.

It will be difficult to parse EPUB files from stdin and the usefulness is
questionable.

src/main.cpp

@@ -84,12 +84,6 @@ int main(int argc, char *argv[])
     int return_code{EXIT_SUCCESS};
 
     vector<fs::path> input_files;
-    if (opts.input_file.empty())
-    {
-        cout << "NO INPUT FILE\n";
-        // TODO: Read data from stdin.
-        return EXIT_FAILURE;
-    }
     for (const auto &filepath : opts.input_file)
     {
         if (!opts.recursive && !opts.dereference_recursive)

src/options.cpp

@@ -91,7 +91,7 @@ options parse_options(int argc, char *argv[])
 
     po::options_description options_hidden("Hidden options");
     options_hidden.add_options()
-        ("input-file", po::value<std::vector<std::string>>()
+        ("input-file", po::value<std::vector<std::string>>()->required()
          ->value_name("FILE"), "Input file to search.")
         ;
     // clang-format on