when evaluating a cheatsheet ruby script, let it know where __FILE__ is.

No more workarounds mentioned in Kapeli/cheatsheets#31.

Now when generating cheatsheet via `cheatset` command, in the cheatsheet Ruby script, we can reference the current cheatsheet ruby file via `__FILE__`.

See example at chitsaou/cheatsheets@ddf603d601c6b7b90abc1da98d71815117851e11.