Add the git_checkout rule

It creates a branch before checking-out to it if the branch does not
exist.

README.md

@@ -146,6 +146,7 @@ using matched rule and run it. Rules enabled by default:
 * `cp_omitting_directory` – adds `-a` when you `cp` directory;
 * `fix_alt_space` – replaces Alt+Space with Space character;
 * `git_add` – fix *"Did you forget to 'git add'?"*;
+* `git_checkout` – creates the branch before checking-out;
 * `git_no_command` – fixes wrong git commands like `git brnch`;
 * `git_push` – adds `--set-upstream origin $branch` to previous failed `git push`;
 * `has_exists_script` – prepends `./` when script/binary exists;