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

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

Currently it's only watching for changes to `.html` and `.md` files located in the root of the project. I was frustrated when livereload stopped working for me: turns out it was because I put my content into subfolders.

refactor hash parsing, fixes issue with autoplaying media not starting from internal links on mobile devices