speed up livereload

[t-fritsch]

livereload in latest version is a lot slower than in previous versions : one has to wait 1-4s when saving html or md file to see the result in the browser.

This PR aims to get back to instant refresh, as it was in previous versions of Reveal (as on 3.6, 3.9, 4.0).

The reason of the drop in performance is this commit : 8492b82#diff-25789e3ba4c2adf4a68996260eb693a441b4a834c38b76167a120f0b51b969f7R289

gulp.task('reload', () => gulp.src(['**/*.html', '**/*.md'])
    .pipe(connect.reload()));

By passing **/*.html and **/*.md it looks like (I'm no gulp expert) we force gulp to crawl recursively every folder in the project, searching for html or md files. And this makes a lot of files even in the base reveal.js project (node_modules is not excluded 😰).

This crawling seems quite useless because this task is not responsible for the watch, all it does is telling the browser to refresh. Still, the connect.reload() API needs a stream of files to execute (I tried many ways to launch it without gulp.src(...) before with no success).
Since the src stream is irrelevant, I made gulp.src only crawl the original index.html file, and everything works fine : when modifying a file, even if it's not the base index.html file, the browser refreshes instantly, and shows every modifications made.

Here is an approximative bench on my dev computer (macbook pro M1Pro, RAM 32Go), measuring time from file save, to modification shown in browser (may differ from timing showed in terminal) :

• old config : 1000-3000ms, thousands (!!!) of network request fired from the browser (all cancelled) 😱
• new config : 10-50ms, no additional network requests 🥳

[hakimel]

This delay has annoyed me for a while now but I was not able to track down the cause. Thanks so much for fixing it 🙌

I tested reloading of html and md files in the root and sub directories and all seems to work perfectly.