Enh/freesurfer resume

I have been trying to add automatic resume to the freesurfer interface, modifying a bit the base interface.
There are not many resumable interfaces in nipype but this seems one interfaces that you want to resume, so if I got it right interfaces will be resumed if _can_resume is True, an _0x*_unfinished.json (does it need to have the hash right?) is in the directory.
It is further checking for existing subject_dir (could be checking for mri/orig/001.mgz or else) and if so then checks for files produced at each step of the freesurfer workflow and generates an alternative command line (that is not the one stored in command.txt) that lists the steps to be skipped or done.
I hope this is working ok.
Also added mris_extract_main_component interface.