I just published the Mark package which is similar to the mark support in Sublime Text and emacs. There is a single mark that can be toggled (cmd-k cmd-k). You can select from the cursor to the mark, jump to the mark, and swap the mark and cursor.
If you’ve never used the mark in other editors, it is very handy for selecting large parts of files, particularly when some portion is off screen. Set the mark where you want the selection to begin then navigate to where you want the selection to end easily without worrying about losing the selection. When you find the end, simply press
cmd-k cmd-a to select between them.