Add a CLI parameter to pass custom values for variables

[akordowski]

Add a CLI parameter to pass custom values for placeholder variables.

Detailed Description

With a CLI parameter custom values could be passed to replace placeholder variables in a text.

Context

In the current yaml config it is only possible to use current owner, repository, milestone variables as placeholder. E.g. in the config close: issue-comment property the {milestone} placeholder is used for the version number. As below defined (example from the Cake project) the links wouldn't work, because the valid v0.1.0 milestone is not a valid version number for the NuGet or Chocolatey packages. So it be good to have a parameter to pass custom values for placeholder.

- [GitHub Release](https://github.com/{owner}/{repository}/releases/tag/{milestone})
- [NuGet Package](https://www.nuget.org/packages/Cake/{milestone})
- [Chocolatey Package](https://chocolatey.org/packages/cake.portable/{milestone})
- [.Net Global Tool](https://www.nuget.org/packages/Cake.Tool/{milestone})

Possible Implementation

I suggest following implementation

• --variables var1=value1;var2=value2
  A semicolon or comma separated list of variables wit values
• Or as a alternative --placeholders var1=value1;var2=value2

I would like to implement the changes and provide a PR.

[gep13]

@akordowski this sounds like a great addition to me!

[akordowski]

Great, then I will go to work the next days.

[gep13]

@akordowski I really think that this is a good idea, so I would be happy if you wanted to spend some time implementing this.

[gep13]

While I think this is a good idea, I don't want to hold up the next release while waiting on this to be implemented, so going to move this to a future milestone.