0.7.1: Delicate Cycle

Yay, custom rules! 🎉

Thanks to @norio-nomura, @scottrhoyt, @Argent & @diogoguimaraes for making this release possible!

Breaking

• None.

Enhancements

• Improve performance of MissingDocsRule.
  Norio Nomura
• Added CustomRules.
  Scott Hoyt
  #123
• Added opt-in ForceUnwrappingRule to issue warnings for all forced
  unwrappings.
  Benjamin Otto
  #55

Bug Fixes

• Fix several false positives in ValidDocsRule.
  diogoguimaraes
  #375