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, 31 insertions, 0 deletions

diff --git a/src/function/external_command.h b/src/function/external_command.h
new file mode 100644
index 0000000..8af5079
--- /dev/null
+++ b/src/function/external_command.h
@@ -0,0 +1,31 @@
+#ifndef INPUTXSLT_SRC_FUNCTION_EXTERNAL_COMMAND_H_
+#define INPUTXSLT_SRC_FUNCTION_EXTERNAL_COMMAND_H_
+
+#include <boost/filesystem.hpp>
+#include <boost/optional.hpp>
+
+#include "base.h"
+
+namespace InputXSLT {
+
+class FunctionExternalCommand : public FunctionBase<
+	FunctionExternalCommand,
+	std::string,
+	boost::optional<std::string>
+> {
+	public:
+		using FunctionBase::FunctionBase;
+
+	protected:
+		friend FunctionBase;
+
+		DomDocumentCache::document_ptr constructDocument(
+			std::string,
+			boost::optional<std::string>
+		) const;
+
+};
+
+}
+
+#endif  // INPUTXSLT_SRC_FUNCTION_EXTERNAL_COMMAND_H_