Add dry-run option

[hkdnet]

ref #6

• add dry-run option
  • it shows not only next version but which files to change
  • this behavior does not match written in #6, but it is better, I think
• extact method and memoize version_cr_path
• change Vup::Up's interface
  • bumpup -> run
  • same as Vup::Show

[tbpgr]