[PATCH, MELT] Plugin documentation generation

[Alexandre Lissy]

Hello,

Documentation for the MELT plugin was only produced as .info file, but
the PDF and HTML output were missing. This was due to missing target for
both ; hence the following patch which fixes by defining two targets:
 - %.html
 - %.pdf
Simply calling respectivement $(TEXI2HTML) and $(TEXI2PDF).