Fix tag prefix

[jordanrastrick]

Addresses the issues raised here.

Two fixes to the functioning of tagPrefix:

• If the tagPrefix option is explicitly given in settings, don't override it with the derived prefix from tagName.
• If the ${npm.name} variable is present in tagPrefix, interpolate it's value before passing through to conventional-recommended-bump

Caveats:

• Does not address interpolating any other variable except npm.name (which presumably would be much less common in tagPrefix) - I'm not familiar enough with the release-it architecture to see how to do this well.

• These are conservative changes in that they shouldn't affect people not trying to use these options/variables.

[jordanrastrick]

@webpro Here's my PR as you suggested, let me know if it makes sense to you.

[webpro]

Thanks for the pull request. Do you have a public repo where we can see this in action? We could maybe create a test, but probably better/easier to have something with only real parts and no mocks.

[jordanrastrick]

Sorry @webpro, I'm only using this package in private repositories that can't be shared.