Add support for xtm (Xtremsplit) files

[ci-macpaw]

Originally reported on Google Code with ID 231

alexandre.feblot:

This format seems to gain wider usage.

format "spec":
http://xtremsplit.fr/xtremsplit-developpeurs-format-extension-xtm.html

[ci-macpaw]

SouthsideIrish:

xtremsplit seems to be a French format, and I run into it a lot on a site called
Lets-Look.com. The site has a lot of Asian drama's and when the person doing the
uploading is French then they tend to use it. The only way I have to join them is to
do it is with xtremesplit running in a Windows XP virtual machine. Oh, and the
program is written in French.

[ci-macpaw]

paracelsus:

Well, I'm not so interested in promoting new format, as in supporting formats people
actually use. There are too 
many experimental formats for me to ever have the time to implement them all.

If anyone actually runs into these files in the wild, please post comments here.