Handle whitespace inside closing tags

[tslnc04]

Fixes #65 by adding a skip whitespace before and after the name of a closing tag. Also adds a test to verify that this functions properly, based on the example in #65.

Note that the HTML spec does not allow for whitespace between the </ and the tag name, although this is accepted by these changes to maintain symmetry with the parsing of the start of a tag which started allowing whitespace between < and the tag name in c532c72.