Automating CLI versioning

[sergey3bv]

Change Description

Addressing #790. Added runtime/debug package for automatic versioning. It uses the last git tag to get the current version. Variable appVersion can be used to set the version manually.

I'm not really happy with panic so let me please know what it can be replaced with.

Steps to Test

1. Build the CLI when checked out on a tag
2. Build the CLI when checked out past a tag
3. Build the CLI with the flag -ldflags "-X github.com/btcsuite/btcwallet/build.appVersion=<custom_version>"

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

📝 Please see our Contribution Guidelines for further guidance.

[sergey3bv]

Hey, @Roasbeef, @yyforyongyu, @guggero, could you please take a look at the PR?