First version

gittargets 0.0.1

• Join rOpenSci.
• Rewrite README to motivate the use case.
• Remove workflow diagram.
• Simplify snapshot model diagram.
• Fix the documentation of the ref argument of tar_git_checkout().
• Add a section to the git.Rmd vignette on code merges.
• Allow command line Git tests to run locally on Windows.
• First version.