Add indexer to store & retrieve custom properties to MinVerVersion

Sometimes the MinVer generated version is the starting point to generate other (custom) representations of version(s), which may or may not be SemVer-compliant.

```
var version = MinVer();
string customVersion1 = null; // Initialized via Setup
string customVersion2 = null; // Initialized via Setup

Task("build")
.Does(() =>
{
// use customVersion1
// use customVersion2
});

Setup(context =>
{
customVersion1 = BuildCustomVersion(version, "A");
customVersion2 = BuildCustomVersion(version, "B");
}
```

It would be nice if we could have something like this instead:

```
var version = MinVer();

Task("build")
.Does(() =>
{
// use version["custom1"]
// use version["custom2"]
});

Setup(context =>
{
version["custom1"] = BuildCustomVersion(version, "A");
version["custom2"] = BuildCustomVersion(version, "B");
}
```

Start by locating the MinVerVersion type and its use in the Cake.MinVer add-in. Review how the generated version is exposed to Cake builds, then determine how indexed custom properties should be stored and retrieved. Done means the issue's version["custom1"] and version["custom2"] examples work for custom representations.

Written by the indexing model from the issue text.