Now supports backup extension. Some warnings are noted in the usage (about backup extensions - one of which goes for sed in general) and the README.md file. Typo fixed the README.md file (some examples were in error for instance) as well and add additional examples. The new version corresponds to https://github.com/xexyl/sgit/commit/9fccaa8cd680d3ed7cd91516710a5672630b5dff; the README.md file had several new commits.