Adding rfcdiff

No real Makefile cleverness. Can probably be improved.

Creates rfcdiff between last tagged version and latest

The --browse parameter to rfcdiff might be annoying for some. 
Rather than create a rule that depends on $(draft).txt and does some magic to dig up the old version of the draft, generate several rules:
* one to dig up the old .xml or .md file (we support markdown too....)
* one to turn the old file into a .txt file (here you can use the existing pattern rule)
* then finally this, which produces the diff

If the intermediate files are marked .INTERMEDIATE, then the cleanup will take care of itself.