v2.1.0

• Fix creating WebLoader without opts. Merge of
  #524.
• Add hasExtension and removeExtension methods to Environment. Merge of
  #512.
• Add support for kwargs in sort filter. Merge of
  #510.
• Add none as a lexed constant evaluating to null. Merge of
  #480.
• Fix rendering of multiple raw blocks. Thanks Aaron O'Mullan. Merge of
  #503.
• Avoid crashing on async loader error. Thanks Samy Pessé. Merge of
  #504.
• Add support for keyword arguments for sort filter. Thanks Andres Pardini. Merge of
  #510