change cast string
change implementation of starts/endsWith similar to lodash
refactoring ternary
Replace all special characters with a dash

Some characters where just removed. #349
Trim special characters in slugify
Document ascii nature of slugify #349
Merge pull request #347 from stoeffel/perf-with

Performance improvement of starts/endsWith
Add CONTRIBUTING.md
Merge pull request #351 from epeli/contributing

Add CONTRIBUTING.md