Added raw version string property in ReleaseNotes class

[ThatRendle]

This is kind of a follow-on from #885

@Redth's SemVer package is great for parsing strings.
Built-in ReleaseNotesParser is great for extracting strings from a MarkDown file, but ReleaseNotes contains a typed Version property, with anything like -preview3-rc8-0002938 stripped off.
If the Parser included the raw string that the Version was parsed from, we could send it off to SemVer for more complex parsing.

[gep13]

@markrendle I think that this would be a sensible addition. Would you be in a position to submit a PR for this? Would be more than happy to accept it 😄

[ThatRendle]

I'll see what I can do...

[ThatRendle]

Done: #1087

[gep13]

@markrendle said,,,
Done: #1087

Cool, will take a look tonight.

[gep13]

@markrendle thanks again for your contribution to Cake 😄