Support C# format strings in config

[asbjornu]

It would be nice if GitVersion supported arbitrary C# format strings, so it was possible to create custom zero padded version number as such:

assembly-informational-format: '{Major}.{Minor}.{Patch}-{CommitsSinceVersionSource:0000}'

the format strings could also be used for other purposes, but zero padding is probably the most useful one.

Originally posted by @asbjornu in #4153 (comment)