Move preprocessing out of the marker and into the parser

[rbnswartz]

Currently there is a preprocessing function on every marker that helps convert from USFM. This should be moved over into the parser so that when we support USX in the future we don't have to add support at the marker level for USX as it shouldn't care whether the incoming data is in USX or USFM. This could also make it easier to handle the next scripture format to come along.