replace :before pseudo elements with :marker selector

allows to have different marker based on the level of nesting (as in other themes : disc, square, circle)

fix code blocks font when printing pdf

fixes #2867

Fix dracula's theme list-style on sub-items

Update demo.html

switch themes keep current slide

Refactored var to let or const, used strict equality operator

fix typos in variable names

fix scss watch tasks broken on syntax error

speed up livereload

connect.reload needs a stream of files to fire, but this stream is irrelevant here and slows refresh time a lot (from ~2ms to 2000ms here)

when there is a syntax error in a sass file (theme or core) the npm start command used to hang, forcing the user to stop and restart the task to compile again.
this fix allows to keep the start-task watching/compiling even when there is an error :
- the error is displayed in terminal
- the rest of gulp tasks are not called (no reload in the browser)
- the user can edit the scss files to try a fix without the need to stop/restart the `npm start` command

update markdown default notes separator to ignore inline occurances of 'notes:', closes #1915, closes #2762

adds ability to override markdown plugin default options

add support for links to the id of an element nested inside slide

allow theme subfolders

fixes livereload when using root CLI param

fixes hakimel/reveal.js#3231

```js
Reveal.initialize({
    markdown: {
        defaultOptions: {
            verticalSeparator: '\n--\n`,
        }
    }
})

allows custom themes to import files from subfolders inside the `css/theme/source` folder.

in `css/theme/source/custom-theme.scss` we can now do
```scss
@import `custom-theme/controls`
@import `custom-theme/headings`
...
```

may allow perf boost

[Markdown plugin] add line number offset for code sections to markdown