add script to generate changelog

[TylerLeonhardt]

In an attempt to automate more of the release process, here's a script to autogenerate the changelog listing all the commits and if they're from external contributors.

Here's what it outputs:

## v1.8.0
### Monday, June 25, 2018
#### [vscode-powershell](https://github.com/powershell/vscode-powershell)

- have PSES generate the random pipename
- revert transport type
- allow path to named pipes to be passed down
- Update to TypeScript 2.9.x (#1344) (Thanks @rkeithhill!)
- SpecProcId - interactive var replacement supports only string type (#1323) (Thanks @rkeithhill!)
- Switch to named pipes (#1327)
- GitHub issue template tweaks and add PSSA template (#1321) (Thanks @bergmeister!)
- Take advantage of multiple issue templatesShamelessly ripping this off from the PSSA GH repo (#1320) (Thanks @rkeithhill!)
- Change SpecifyScriptArgs command to only return string - not string[] (#1317) (Thanks @rkeithhill!)
- Update package veresion in lock file, format package.json file. (#1318) (Thanks @rkeithhill!)
- Updates to Examples PSSA settings file to include more rule config (#1312) (Thanks @rkeithhill!)
- Make SaveAs work for untitled files (#1305)
- Added Columns, Improved readability for ToC. (#1307) (Thanks @st0le!)
- Added some snippets (#1297) (Thanks @SQLDBAWithABeard!)

#### [PowerShellEditorServices](https://github.com/powershell/PowerShellEditorServices)

- add what to do when there's a vulnerability to docs (#687)
- Fix issue where MS Dynamics CRM (#686) (Thanks @rkeithhill!)
- Make AnalysisService use the latest version of PSScriptAnalyzer (#677)
- Fix PSES crash that happens if you format an empty PS doc (#685) (Thanks @rkeithhill!)
- Include ThirdPartyNotices.txt to comply with OSS guidelines
- Initial CODEOWNERS file to auto assign PR reviewers (#681) (Thanks @rkeithhill!)
- Add more useful PSSA rules that should be enabled by default (#669) (Thanks @bergmeister!)
- Add symbols to modules built in Debug configuration (#675)
- Implement initialized notification handler to get rid of log error (#674) (Thanks @rkeithhill!)
- Change logging to use Serilog (#666)
- Stop before we restart (#668)
- Add .gitattributes, .editorconfig and extensions.json (#667) (Thanks @rkeithhill!)
- ignore .idea folder that jetbrains products like to spit out (Rider, IntelliJ, Resharper) (#664)
- Close stray processes on exit (#663)

[rjmholt]

Wow! This is awesome!

[TylerLeonhardt]

tbh, stole most from the PowerShell repo :)

https://github.com/PowerShell/PowerShell/blob/master/tools/releaseTools.psm1#L129

[rkeithhill]

LGTM. I'm all for automation!

[rjmholt]

LGTM!

[rjmholt]

We should probably document any of the bits we need to make this work somewhere in the repo (like in the README). The big one I'm thinking of is [Breaking Change].

[rjmholt]

But also just making sure the final, post-squash commit message looks right in the changelog

[TylerLeonhardt]

@rjmholt yeah the result of this script is in addition to anything else we want to highlight in the Changelog.

Also, if it's not on already, we should force squashing commits so that the PR number is in every commit.

[TylerLeonhardt]

I turned on forcing Squash.