Add litable-result-format customization variable

A format string for printing the result in the overlay. Defaults to " %s ".
Add litable-result-face customization varioble

Defaults to inheriting from font-lock-warning-face
Add litable-create-result-overlay-function customization variable
Typo fix
Update README
Renaming result-overlay-text var and function
Add litable-substitution-face customization variable
Add litable-substitution-overlay-text-function customization
Minor changes to README
Merge pull request #15 from alvinfrancis/master

Add customisation variables