Add optional startFilePos and endFilePos attributes

The lexer can now optionally add startFilePos and endFilePos
attributes, which are offsets in to the lexed code string.

The end offset currently points one past the last character of
the token - this is pending further discussion.

The attributes are not added by default and have to be enabled
using the new 'usedAttributes' lexer option:

	$lexer = new Lexer([
		'usedAttributes' => [
			'comments', 'startLine', 'endLine',
			'startFilePos', 'endFilePos'
		]
	]);
Support file offsets in emulative lexer as well

Also run normal lexer tests against emulative lexer and fix a bug
in __halt_compiler() handling found as a result.