Support older version of docopt.

Older version of docopt doesn't define Options.

Let's define it using `using` syntax (as done in recent version of docoptcpp)

src/zimcheck/zimcheck.cpp

@@ -76,6 +76,9 @@ R"(Zimcheck checks the quality of a ZIM file.
  zimcheck -m --favicon wikipedia.zim)";
 
 
+// Older version of docopt doesn't define Options
+using Options = std::map<std::string, docopt::value>;
+
 template<class T>
 std::string stringify(const T& x)
 {
@@ -97,7 +100,7 @@ int zimcheck(const std::vector<const char*>& args) {
         args_string.emplace_back(arg);
     }
 
-    docopt::Options parsed_args;
+    Options parsed_args;
     try {
       parsed_args = docopt::docopt_parse(
         USAGE,
@@ -112,7 +115,7 @@ int zimcheck(const std::vector<const char*>& args) {
     return zimcheck(parsed_args);
 }
 
-int zimcheck(const docopt::Options& args)
+int zimcheck(const Options& args)
 {
     // To calculate the total time taken by the program to run.
     const auto starttime = std::chrono::steady_clock::now();

src/zimdump.cpp

@@ -386,7 +386,9 @@ Return value:
       See DIR/dump_errors.log for the listing of the errors.
 )";
 
-typedef std::map<std::string, docopt::value> Options;
+
+// Older version of docopt doesn't define Options
+using Options = std::map<std::string, docopt::value>;
 
 int subcmdInfo(ZimDumper &app, Options &args)
 {

src/zimsplit.cpp

@@ -180,10 +180,10 @@ int main(int argc, char* argv[])
   {
     std::ostringstream versions;
     printVersions(versions);
-    std::map<std::string, docopt::value> args = docopt::docopt(USAGE,
-                                                               {argv + 1, argv + argc},
-                                                               true,
-                                                               versions.str());
+    auto args = docopt::docopt(USAGE,
+                              {argv + 1, argv + argc},
+                              true,
+                              versions.str());
 
     std::string prefix = args["<file>"].asString();
     if (args["--prefix"])