Q2A comes with CKEditor which allows posting HTML. If you want to be able to type actual HTML code instead of a visual editor like CK then there are 2 options:
1. Add the Source button to CKEditor. This is done by editing qa-plugin/wysiwyg-editor/ckeditor/config.js and adding 'Source' to one of the lists under config.toolbar
2. Create your own editor plugin (see docs). You can copy qa-include/plugins/qa-editor-basic.php and set the format to 'html'. That should give you a plain text box to enter HTML in.