fix #17: wrong color string parsing
use BufEnter instead of BufRead

so switching buffers in the same window results in updated highlighting
matchdelete may fail

fixes github issue #15
fix toggling not work; fixes #18
fix make uninstall, closing #19
two some tweaks

min() comparing to if-condition, has almost the same perfermance,(in
fact, min() is tiny faster than if-condition in most test cases), and
min() looks more clear
Only colorize visible area; remove a splith autocmd

Faster for large file
Move autocmd outside