Updated Prosession

* Updated the GetCurrDirName function to append the SHA256 hash to the
  current directory name instead of just using the SHA256. This is for
  cosmetic reasons and makes it tad easier while using completion with
  `:Prosession` command to complete existing session names to switch to.
* Limited SHA256 hash code length by using g:prosession_sha_length
  variable having default value of '8' to limit the session file name
  size.
* Added a delimiter of `__sha256__` between the current directory name
  and it's sha256 hash code for use with updating tmux window name if
  you have enabled g:prosession_tmux_title to limit the tmux window name
  to just the directory name.