[Feature] Add Flag aware Release Notes

### SUMMARY

The current Release notes feature in the Mobile.BuildTools while configurable is also "dumb". It will simple create release notes based on the commits over the specific timeframe and other limits defined. We should be able to create more customized and dynamic Release Notes for each platform.

### DETAILS

The goal here will be to be able to create release notes since the last Git Tag or a Specified commit id. We want to look only at messages that begin with a [Flag]. Flags may be either a platform tag or some other element. We will want this to be customizable by allowing you to provide a custom Release Notes template and will provide one like:

```
# Release Notes

## Features

{{Feature}}

## Bug Fixes

{{Bug}}
```

This template would then look for commit messages like:

- [Feature] Some awesome new thing we did
- [Bug] Some pesky thing we fixed

We will further want to evaluate the platform so that platform specific issues are only included when the Release Notes are generated for that platform like the following should only be included in the release notes for the iOS build:

- [Feature][iOS] Something neat we did

### PLATFORMS

- [x] Android
- [x] iOS
- [x] UWP