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, 2 insertions, 2 deletions

diff --git a/README.md b/README.md
index 8629b9a..0593bb5 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ Contrary to popular opinion I actually like XSLT as a content transformation lan
 - external `read-directory` function for read-only directory traversal
 - external `transform` function for executing transformations inside transformations
 - external `generate` function for executing transformations and committing the result directly to the filesystem
-- external `external-text-formatter` function for executing text formatters and capturing their XML output
+- external `external-command` function for executing external commands such as text formatters and capturing their output
 - external `write-file` function for writing files
 
 ## Examples:
@@ -24,7 +24,7 @@ The `test` directory contains black-box test cases for every external function p
 - [`InputXSLT:read-directory`](test/read_directory/transformation.xsl)
 - [`InputXSLT:transform`](test/transform/transformation.xsl)
 - [`InputXSLT:generate`](test/generate/transformation.xsl)
-- [`InputXSLT:external-text-formatter`](test/external_text_formatter/transformation.xsl) (requires [markdown.pl](http://daringfireball.net/projects/markdown/))
+- [`InputXSLT:external-command` (text formatting)](test/external_text_formatter/transformation.xsl) (requires [markdown.pl](http://daringfireball.net/projects/markdown/))
 - [`InputXSLT:write-file`](test/write_file/transformation.xsl)
 
 Concepts of how static sites may be generated using InputXSLT are being evaluated in [TestXSLT](https://github.com/KnairdA/TestXSLT).