Allow for indefinite nested lists

Improve nested list processing

Add processing for < and > as it gets escaped and seen as XML t…

…ag open/close

Fixed empty list (item) processing

Added better support for nested lists

Decided to keep allowing for PHP 5

Add "support" for nested lists

Nothing fancy, just makes it not break. Not possible to make actual
nested lists without access to `numbering.xml`

Add _removeStartSpaces

Remove unnecessary spacing at the beginning

Fix ampersands

Make sure ampersands are trailed by "amp;"

Minor changes

Updated composer.json and readme