[Feature] Add an additional command line argument - "-format" to allow to output an expression instead of a singular version variable

[arturcic]

Currently there is a -showvariable switch that allows us from command line to return a specific version variable.
By adding a new -format switch, we are able to customize the way the output looks like, allowing to combine several version variables and environment variables.

For example:

`dotnet gitversion -format "{Major}.{Minor}" will output only these 2 variables combined.

[arturcic]

🎉 This issue has been resolved in version 6.0.0-beta.1 🎉
The release is available on:

• GitHub Release
• Chocolatey Package
• .Net Global Tool

Your GitReleaseManager bot 📦🚀