Introduce DEFAULT_CHEAT_DIR environment variable

Now ~/.cheat can be overridden
Remove odd new line
Add note about DEFAULT_CHEAT_DIR to the README file
Merge branch 'DEFAULT_CHEAT_DIR' of https://github.com/a-sk/cheat into a-sk-DEFAULT_CHEAT_DIR

* 'DEFAULT_CHEAT_DIR' of https://github.com/a-sk/cheat:
  Add note about DEFAULT_CHEAT_DIR to the README file
  Remove odd new line
  Introduce DEFAULT_CHEAT_DIR environment variable
Modified the README.