It's a shame Textreme doesn't have this built-in, but you can set it up yourself!
Make a file named "Open With Textreme.cmd" (or whatever you like, as long as it's a CMD file), and write this in it:
"C:\Users\Tophwells\Documents\Textreme\Textreme2.exe" --target_path=%1
replacing the part between quotes with whatever the path is to your copy of Textreme. Then set text files to open with this file instead.
There'll be a little command window that pops up every time you start Textreme in this way, but you can ignore it. Close it if it's bothering you.