Added version tagging for master commits

[Zozman]

Currently, it is extremely difficult to audit changes in this repository due to the fact that there is no sort of tagging when code is committed to the master branch of this repository. Therefore there is no way to know what exact version of ngx_brotli your using when downloading from GitHub unless you use a SHA of some sort, which is unwieldy.

Therefore this PR adds a GitHub Action to update a tag on the repo every time a commit is made to master. That way there is a tag to reference on each change in master, allowing for easier audibility of the repository.

Also resolves issues pointed out around tagging in Issue #120.

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[PDowney]

This is the #1 reason why I refuse to implement this module into my Nginx builds. Without some type of versioning control, it's a roll of the dice as to whether the version that you installed is stable. It probably doesn't matter too much anyway though, as this module hasn't been updated since April.

[Zozman]

I've borked up the commit history on this branch so have re-submitted it with a much cleaner commit history in #144.