Support alternate tag prefixes

[Gudahtt]

We should support alternate prefixes in version tags. At the moment the project assumes that the prefix v is always used. But some projects use no prefix, or include the package as a prefix to the version (e.g. in monorepos).

All public-facing functions now support a tag prefix option. It is optional, so this is a non-breaking change. The updateChangelog
method accepts an array of tag prefixes instead of a single one, so that we can better support the first release of a monorepo where you might want to compare against a backup tag if the first isn't found.

Fixes #116

[Gudahtt]

I will follow up with manual testing steps soon, and do some manual testing myself.

[Gudahtt]

Manual testing steps:

Independent repository (regression testing):

• Checkout auto-changelog and build the project
• Add the script `"auto": "[path-to-auto-changelog-project]/dist/cli.js" to some repository for test purposes
  • It can be any of our projects, as long as it's not a monorepo, and as long as it has some changes and some previous git tag.
• Run yarn auto update --rc
• See that it correctly identified the changes since the last tag, and it correctly set the release header in the changelog.

Monorepo repository (new functionality):

• Checkout auto-changelog and build the project
• Checkout the controllers repository monorepo-poc branch, and build the project
• Add the script `"auto": "[path-to-auto-changelog-project]/dist/cli.js" to the root of the repository.
• Set the current base-controller version to 1.0.0
• Run yarn auto update --rc --root ./packages/base-controller --tag-prefix=base-controller@v --currentVersion 1.0.0
• See that it correctly identified the changes since the last tag, and it correctly set the release header in the changelog.
• Update the draft changelog place all changes into the "Changed" category, making it pass validation.
• Update the current base-controller version to 1.1.0
• Commit the changelog changes and tag that commit as base-controller@v1.1.0
• Make some example commits that alter the base controller package
• Run yarn auto update --rc --root ./packages/base-controller --tag-prefix=base-controller@v --currentVersion 1.1.0
• See that it correctly identifies just the example commits

[Gudahtt]

The manual testing steps include setting the --currentVersion flag because the default version used is npm_project_version, which is wrong if the update script is being run from the root of the monorepo.

This seems wrong to me (the version should always match the manifest at the project root), so I have updated how that works here: #121

[rickycodes]

This all LGTM