Add message types to spectags

Command to generate spectags file:
cd i2p2www/pages/site && ctags -f spectags --regex-html=/\"type_\([a-zA-Z0-9]+\)\"/\\1/t,type/ --regex-html=/\"struct_\([a-zA-Z0-9]+\)\"/\\1/s,struct/ --regex-html=/\"msg_\([a-zA-Z]+\)\"/\\1/m,msg/ -R -n docs/spec/*.html