v3.1.0

• This release swaps postcss-single-charset for postcss-normalize-charset,
  which can detect encoding to determine whether a charset is necessary.
  Optionally, you can set the add option to true to prepend a UTF-8
  charset to the output automatically (thanks to @TrySound).
• A safe option was added, which disables more aggressive optimisations, as
  a convenient preset configuration (thanks to @TrySound).
• Added an option to convert from deg to turn & vice versa, & improved
  minification performance in functions (thanks to @TrySound).