Hook before import

Hey @tobiasbg

Thanks for the plugin.
This is a very useful plugin, but I meet some troubles with the import of the HTML.
The problem in using invalid markup in the HTML files which provided by another system and problem can’t be done without modification $temp_data in the class-import.php (line 158). Each time when I uploading the file with the invalid markup libxml will return an error (and this is okay) but this will be much more powerful if we will modify the $temp_data before starting working with it in the next step. Something like $temp_data = apply_filters('tablepress_html_import_temp_data', $temp_data); will be enough I think.

Thanks, Vlad.