0.15.0

• (breaking change) ! has now its meaning hardcoded in the language. If you defined it for a type it won't be invoked as a method anymore.
• (breaking change) nil? has now its meaning hardcoded in the language. If you defined it for a type it won't be invoked as a method anymore.
• (breaking change) typeof is now disallowed in alias declarations
• Added crystal tool format --check to check that source code is properly formatted
• crystal play (playground) added workbooks support, as well as improvements and stabilizations
• Added Tempfile.dirname (thanks @DougEverly)
• Added Path#resolve method in macros
• {{...}} arguments to a macro call are now expanded before macro invocation (#2392)
• Special variables ($~ and $?) are now accessible after being defined in blocks (#2194)
• Some bugs and regressions fixed