Add `cljr-cycle-thread` for toggling between thread first/last

Stray p in the code

Introducing let binding

Replacing kill-sexp with delete-region to keep it out of the kill ring

Default keybinding for cljr-cycle-thread -> C-! ct

2 tests for cljr-cycle-thread
Merge pull request #71 from lsnape/toggle-thread-position

Add `cljr-cycle-thread` for toggling between thread first/last