Allow user to choose which function prints her data

* litable.el (litable--print-result):
(litable--print-input):
(litable--create-substitution-overlay): replace printing function by
variable litable-print-function

(litable-print-function): new defcustom, holds the function name to
print the data, defaults to pp-to-string
Merge pull request #10 from YoungFrog/master

Customizable print function