Clara cleanups

* Clara is now split between a header and a cpp file.
* Removed the deprecated `+` and `+=` operators for composing
  a parser.
* Renamed `clara` and `detail` namespaces to be inline with the
rest of Catch2 (they are now `Clara` and `Detail` respectively).
* Taken most of user-exposed types out of the `Detail` namespace
completely (instead of using `using` directives to bring them into
the outer namespace).

src/catch2/catch_session.cpp

@@ -176,7 +176,7 @@ namespace Catch {
         if( m_startupExceptions )
             return 1;
 
-        auto result = m_cli.parse( clara::Args( argc, argv ) );
+        auto result = m_cli.parse( Clara::Args( argc, argv ) );
         if( !result ) {
             config();
             getCurrentMutableContext().setConfig(m_config.get());
@@ -239,10 +239,10 @@ namespace Catch {
         return exitCode;
     }
 
-    clara::Parser const& Session::cli() const {
+    Clara::Parser const& Session::cli() const {
         return m_cli;
     }
-    void Session::cli( clara::Parser const& newParser ) {
+    void Session::cli( Clara::Parser const& newParser ) {
         m_cli = newParser;
     }
     ConfigData& Session::configData() {