fix typo.
Added opacity variables to `slick` settings.

Opacity settings could not be overridden.
Adding contribution guidelines.
Update CONTRIBUTING.markdown
Fixing #268 & #647 & 655
Fixing #643
Fixing #640
Fixing #480
Fixing #669
Fixing #665 , #638
Merge pull request #623 from ORiON-/master

fix typo.
Fixing #650
Merge branch 'master' of https://github.com/kenwheeler/slick into 1.3.12
Fixing #500
Merge pull request #644 from gwa/master

Added opacity variables to `slick` settings.
Fixing style issues
Fixing link navigate on non slide changing swipe
Fixing #697
Compacting and adding source map
Fixing #676 & #574