0.10.0: FailableKitten

This release is all @norio-nomura, thanks!

Breaking

• Change SwiftDocs.init(file:arguments:) to
  SwiftDocs.init?(file:arguments:).
  Norio Nomura

Enhancements

• Add Request.failableSend() that can handle SourceKitService crashes.
  sourcekitten doc does not stop when SourceKitService crashes.
  Norio Nomura

Bug Fixes

• Fix crash when offset points end of string.
  Norio Nomura
  #164