Version 0.1.3: Fixes and language extensions

This release sees one language extension, a new function, one optimization, and bug fixes. Huge thanks to @vectro for contributions.

Language changes:

• Added if filters in array and object comprehensions
• The flatten() function was added

Optimizations:

• Tiny optimization in object comprehensions

Bug fixes:

• Implemented let inside object comprehensions
• Fix round-tripping bug in date parsing/formatting (resolved #8), thanks to @vectro
• Bug fix and improvement to the number() function, thanks to @vectro
• Bug fix in date parsing/formatting (2 milliseconds off), thanks to @vectro