v1.0.0-alpha3

🚀 New feature:

• Issue #70 upgraded bootstrap to v4.0.0 and updated docs
• Issue #27 created 'best practices' document
• Issue #73 added mixin for placeholder in forms mixins
• Issue #75 added mixins for buttons
• added gradient-bg mixin from bootstrap
• added border radius mixins

🐛 Fixed:

• fixed links in readme for gradients
• Issue #69 changed $grid-gutter-widths variable in mixin files
  and _variables.scss
• fixed buttons documentation
• fixed border-radius documentation and variables
• Issue #80 fixed media-breakpoint-variant mixin, variables and documentation
• Issue #80 removed absolute property from pseudo mixin in
  positioning documentation
• Issue #80 added missing px in responsive-prop mixin in
  responsive documentation
• Issue #80 changed hardcoded colors to variables in documentation