v0.6.0

• fixes stack-space mixin
• fixes heading-xsmall mixin
• adds global mixin:
  • caption-xsmall (was missing)
  • separator to create top/bottom hr-like separator with border
  • truncate-text, does what it says
• adds global variable:
  • $text-max-width
• adds marketplace variables:
  • $block-card-shadow
  • $container-max-width
  • $container-padding
  • $separator style (used in mixin)