v0.4.0

Added

• Add a new canvas shape: Rectangle.
• Official support of Crossterm backend.
• Make it possible to choose the divider between Tabs.
• Add word wrapping on Paragraph.
• The gauge widget accepts a ratio (f64 between 0 and 1) in addition of a
  percentage.

Changed

• Upgrade to Rust 2018 edition.

Fixed

• Fix rendering of double-width characters.
• Fix race condition on the size of the terminal and expose a size that is
  safe to use when drawing through Frame::size.
• Prevent unsigned int overflow on large screens.