Update get_pref() to allow settings inside Sublime project files

[TomAnthony]

I use this plugin, but across an array of projects, some of which I want to be prettified on save, and others I don't. I use .sublime-project files and many related plugins (e.g. isort, sublack) allow settings entries in that file for per-project settings.

This PR brings that functionality to Sublime-HTMLPrettify, allowing settings in .sublime-project files (with a prefix).

For example if you want to update the format_on_save in a .sublime-project file you can add in:

"settings":
{
	"HTMLPrettify.format_on_save": true
}