0.3.0: Compile subcommand

• Registered xp compile subcommand - @thekid
• Simulated yield from in PHP 5.6 in a limited fashion - @thekid
• Added support for nullable types from PHP 7.1 - @thekid
• Implemented short list(...) syntax from PHP 7.1 - @thekid
• Added support for anonymous classes from PHP 7.0 - @thekid
• Implemented constant modifiers from PHP 7.1 - @thekid
• Added support for comparison operator <=> from PHP 7.0 - @thekid
• Added support for object typehint from PHP 7.2 - @thekid