perf: limit MatchTagAlways to certain filetypes
perf: limit MatchTagAlways to certain filetypes

MatchTagAlways significantly slows down startup, even on filetypes where
it isn't configured to do anything. This uses vim-plug's on-demand
loading feature to avoid taking the performance hit on other filetypes.

Test: vim --startuptime vim.log file.cc (MatchTagAlways absent from log)
Test: vim file.html (MatchTagAlways does its job)

nfischer/vimfiles