Add experimental plugin system

Plugins define the interface specified in plugin.h and have
full access to any functions exposed in mkxp.
Specify plugins to be loaded in the config file via
'plugin=/path/to/plugin.so' lines.

A sample plugin for the MRI binding is provided.

sample-plugin/main.cpp

@@ -0,0 +1,41 @@
+
+#include "plugin.h"
+#include "debugwriter.h"
+#include "exception.h"
+
+#include "binding-util.h"
+
+static void pluginInit();
+static void pluginFini();
+
+Plugin plugin =
+{
+    pluginInit,
+    pluginFini,
+    PLUGIN_MRI
+};
+
+/* Callable from ruby scripts as 'plugin_function' */
+RB_METHOD(pluginFunction)
+{
+	RB_UNUSED_PARAM;
+
+	const char *str = 0;
+	rb_get_args(argc, argv, "|z", &str RB_ARG_END);
+
+	Debug() << "Sample plugin function:" << str;
+
+	return Qnil;
+}
+
+void pluginInit()
+{
+	_rb_define_module_function(rb_mKernel, "plugin_function", pluginFunction);
+
+	Debug() << "Sample plugin inited!";
+}
+
+void pluginFini()
+{
+	Debug() << "Sample plugin finited!";
+}

sample-plugin/sample-plugin.pro

@@ -0,0 +1,18 @@
+######################################################################
+# Automatically generated by qmake (2.01a) Fri Jan 3 01:40:45 2014
+######################################################################
+
+TEMPLATE = lib
+QT =
+TARGET = sample-plugin
+DEPENDPATH += . ../src ../binding-mri
+INCLUDEPATH += . ../src ../binding-mri
+CONFIG += plugin
+
+unix {
+	CONFIG += link_pkgconfig
+	PKGCONFIG += ruby-2.1
+}
+
+# Input
+SOURCES += main.cpp