fixed undecoded UTF-8 warning from HTML::Parser in test cases

test_cases.t was producing the following warning
Parsing of undecoded UTF-8 will give garbage when decoding entities at
[...]/5.16.3/HTML/TreeBuilder.pm line 108.

Fix is to change the call to File::Slurp to read the HTML data in
utf-8 mode.
Must import blessed() for HTML::TreeBuilder input.
added 'output_type' option and tests

Valid options are 'xhtml', 'html' and 'tree'. If not provided, the
original XHTML format will be returned. If 'tree' is selected,
extract_main_html will return an HTML::Element object.