Better Tables on legacy Windows terminals

A new major version due to changes to Rule and Panel constructor, and Table.add_column method. This is unlikely to break anyones code but semver mandates a major version bump.

Otherwise, the highlight of this release is that legacy Windows supports a greater range of box characters for Table / Panel etc.

[3.0.0] - 2020-06-28

Changed

• Enabled supported box chars for legacy Windows, and introduce safe_box flag
• Disable hyperlinks on legacy Windows
• Constructors for Rule and Panel now have keyword only arguments (reason for major version bump)
• Table.add_colum added keyword only arguments

Fixed

• Fixed Table measure