Implemented primitive optional parameter support for external functions

* renamed FunctionExternalTextFormatter into FunctionExternalCommand
** the goal is to provide a general interface to a variety of external commands
*** e.g. not just text formatters but system utilities for file management and so on
** this requires the stdin parameter to be optional as not all external commands require stdin input
* implemented "filter_derived" helper template to determine amount of optional parameters
** optional parameters are defined as "boost::optional" specializations
*** they in turn can be detected by checking if "boost::optional_detail::optional_tag" is a base class
* "callConstructDocument" member method of "FunctionBase" performs additional bounds checking of parameter vector
* "boost::optional<std::string>" specific member overload was added to "XObjectValue" helper class
** we will have to provide full specializations for all optional types as C++ prohibits partial member function template specialization
* renamed the external function in "PlattformGuard"
* changed README.md and test cases accordingly

1 files changed, 3 insertions, 3 deletions

diff --git a/src/platform_guard.cc b/src/platform_guard.cc
index 6be5b4a..f50d976 100644
--- a/src/platform_guard.cc
+++ b/src/platform_guard.cc
@@ -11,7 +11,7 @@
 #include "function/read_directory.h"
 #include "function/transform.h"
 #include "function/generate.h"
-#include "function/external_text_formatter.h"
+#include "function/external_command.h"
 
 namespace InputXSLT {
 
@@ -56,8 +56,8 @@ PlatformGuard::PlatformGuard(const std::vector<std::string>& path):
 
 	xalan::XalanTransformer::installExternalFunctionGlobal(
 		customNamespace,
-		xalan::XalanDOMString("external-text-formatter"),
-		InputXSLT::FunctionExternalTextFormatter(&this->include_resolver_)
+		xalan::XalanDOMString("external-command"),
+		InputXSLT::FunctionExternalCommand(&this->include_resolver_)
 	);
 }