Global install support

This release enables commitplease to be installed globally and be manually executed from console (previously it was always executed automatically as a git hook). Global executable respects repo-local settings and can check more than just the latest commit:

Use case	command
Check all commits on branch master	commitplease master
Check all commits on branch feature that are not on master	commitplease master..feature
Check all commits on current branch that are not on master	commitplease master..HEAD
Check the latest 1 commit	commitplease -1
Check all commits between 84991d and 2021ce	commitplease 84991d..2021ce
Check all commits starting with 84991d	commitplease 84991d..

Here you can read more about git commit ranges. To start using global executable, install it with

npm install -g commitplease