Add script for bumping plugin versions

[tterb]

Description

This adds a new script for bumping the plugin version and creating a git version tag. I originally was going to try to add the git version tag creation to the existing bin/deploy.sh script, but it felt kind of out of place throwing git commands in there since we're only using SVN in that script and it would require adding environment variables for git oauth.

Though, I'm on the fence about what would be the best release workflow:

• To trigger a new release we run ./bin/bump-version.sh and have the script automatically push the version updates and version tag to master with git push --tags origin master.

• To trigger a new release we run ./bin/bump-version.sh, push that to a separate branch (we'd need to remember to include the --tags flag), create a PR, and then the release will go out when that is merged.

Motivation and Context

There are multiple files that need the version updated to trigger the deploy action in the CI, which leaves a lot of room for potential user-error. We also haven't been very good at remembering to push git tags when new versions of the plugin go out, so this also adds the git tag while updating the versions in the other files.

How Has This Been Tested?

Tested the script locally

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This change is 

[tterb]

@thekashifmalik I agree we should definitely rename the script to bin/publish if we have it push the changes to master and I think that was the option that I've been leaning towards as well.

For the disqus-react repo, I will usually just create a PR that updates the changelogs and bumps the package version before publishing an update just because I don't like the idea of pushing directly to master, but that repo also doesn't have a CI that takes 45 mins to run...

[thekashifmalik]

I'm with it. As long as we're cutting the releases I'm okay with pushing to master. It's not perfect, but nothing is.