tool to manage git semantic versioning for git repositories 

the README is basically copy+pasted from npm version, right now

One thing I struggled with was if it was better to exec git directly or use the [official Go bindings for Git](https://github.com/libgit2/git2go).

Pros of official bindings:
  - better Git integration with Go (actual errors vs only exit code + parsing stderr)
  - not dependent on consumer's git version in case of changing git API

Cons of official bindings:
  - requires building and linking libgit2 into the binary
  - lots of commands we don't want or care about, further swelling binary size
This will error with "missing version argument" in a case like `gitsem patch additionargument`. What do you think about printing usage and then `exitWithError("gitsem takes exactly one argument")`?