Add "auto" version modifier

[ian-fox]

This adds another possible value for the version argument: "auto"

If somebody runs cargo bump auto, it will look at their most recent commit message. If that message contains the string [major], it performs a major bump. Similarly for [minor], and if neither is found it defaults to [patch].

Example:

$ grep 'version =' Cargo.toml'
version = "1.1.0"
$ git log -1
commit 86bfa1bf8b70c4ce3a58ae47458d9d2c8d58b279 (HEAD -> auto)
Author: Ian Fox <code@ianfox.io>
Date:   Wed Sep 23 23:11:27 2020 -0400

    [minor] add 'auto' version modifier
$ cargo bump auto
$ grep 'version =' Cargo.toml'
version = "1.2.0"

Did manual testing but didn't add a unit test as it would have meant a bunch of effort to mock the output of the process::Command and that wasn't there for the other git commands.

Also bumped the version of toml_edit becuase it wasn't compiling for me on 0.1.3, removed a .clone() which rustc told me was unnecessary, and renamed the functions in the git module to avoid stutter.