Add make target for other languages

For example, to build the french version of the guide (will output a
file named tutorial-fr.pdf):

    make pdf-lang GUIDE_LANG=fr

Makefile

@@ -1,6 +1,8 @@
 PANDOC = pandoc
 INPUT = README.md
+INPUT_LANG = guide-$(GUIDE_LANG).md
 OUTPUT = tutorial.pdf
+OUTPUT_LANG = tutorial-$(GUIDE_LANG).pdf
 TITLE = -V title=LearnHaskell
 AUTHOR = -V author=bitemyapp
 FONTSIZE = -V fontsize=12pt
@@ -13,5 +15,8 @@ FLAGS = --normalize --smart --toc $(VARIABLES)
 pdf: README.md
 	$(PANDOC) -s $(INPUT) -o $(OUTPUT) $(FLAGS) --column=80
 
+pdf-lang: guide-$(GUIDE_LANG).md
+	$(PANDOC) -s $(INPUT_LANG) -o $(OUTPUT_LANG) $(FLAGS) --column=80
+
 dialogues: dialogues.md
 	$(PANDOC) -s dialogues.md -o dialogues.pdf $(FLAGS) --column=80