Describe scenario when pattern token is followed by a colon

Closes #620
Describe scenario about parameter with decimal number following string

Covers #615
Fix broken scenarios by switching to nehative regex looks

Fixes #615
Describe scenario where pattern parameter is empty

Covers #614
Fix scenario where pattern parameter is empty

Fixes #614
Describe scenarios where steps use slashes

Slashes are alternation syntax in Turnip. Thus, both snippet generation
and pattern matching are broken in steps that have them.

Covers #598
Fix scenarios where steps have slashes

Fixes #598
Merge pull request #622 from Behat/bugfix/turnip-patterns

[bugfix] Turnip patterns edge cases