2.1.0

• Bumped css-tree to ^2.3.1 (#47, #48)
• Extended ignoreAtrules and ignoreProperties options to accept RegExp patterns (#19, #45)
• Fixed Sass's @else at-rule to allow have no a prelude (#46)
• Changed at-rule prelude validation to emit no warnings when a prelude contains Sass/Less syntax extensions (#44)