slug: Add more validations to IsSlug

The new validations are for empty string, _- as prefix or sufix of the slug and MaxLength validation if setted

add: IsSlug function

Fix gosimple#11

Add LICENSE file

update: lint fixes for docs

change: always apply Substitute map in alphabetic order

fix gosimple#8

fix: broken Substitute

Substitution did not work if substitution map had more than one element.

fix gosimple#6