Change changelog script to use PSES syntax

[rjmholt]

PR Summary

Changes the changelog generation script so the output doesn't need rewriting on releases.

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[TylerLeonhardt]

Can you post the output for reference?

[TylerLeonhardt]

LGTM :)

[rjmholt]

Example:

> .\tools\Get-PowerShellExtensionChangelog.ps1 -LastReleaseTag 'v1.8.2' -NewReleaseTag 'v1.8.3' -Token $myToken

## v1.8.3
### Thursday, August 16, 2018
#### [vscode-powershell](https://github.com/powershell/vscode-powershell)

- [vscode-PowerShell](https://github.com/PowerShell/vscode-PowerShell) -
  Change changelog script to use PSES syntax
- [vscode-PowerShell #1483](https://github.com/PowerShell/vscode-PowerShell/pulls/1483) -
  Update CHANGELOG for v1.8.3
- [vscode-PowerShell #1480](https://github.com/PowerShell/vscode-PowerShell/pulls/1480) -
  Use PowerShell signing script in VSTS builds
- [vscode-PowerShell #1460](https://github.com/PowerShell/vscode-PowerShell/pulls/1460) -
  Use newer version for preleases
- [vscode-PowerShell #1475](https://github.com/PowerShell/vscode-PowerShell/pulls/1475) -
  Change resourceLangId to editorLangId so right-click works properly with unsaved files  (Thanks @corbob!)
- [vscode-PowerShell #1467](https://github.com/PowerShell/vscode-PowerShell/pulls/1467) -
  (GH-1458) Remove region folding from non-region areas  (Thanks @glennsarti!)

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

- [PowerShellEditorServices](https://github.com/PowerShell/PowerShellEditorServices) -
  Increment version number
- [PowerShellEditorServices #723](https://github.com/PowerShell/PowerShellEditorServices/pulls/723) -
  Update CHANGELOG for v1.8.3
- [PowerShellEditorServices #722](https://github.com/PowerShell/PowerShellEditorServices/pulls/722) -
  Add VSTS signing step
- [PowerShellEditorServices #717](https://github.com/PowerShell/PowerShellEditorServices/pulls/717) -
  increment version for prerelease
- [PowerShellEditorServices #715](https://github.com/PowerShell/PowerShellEditorServices/pulls/715) -
  Reduce allocations when parsing files  (Thanks @mattpwhite!)

[TylerLeonhardt]

sweeeeeeeeet

[dragonwolf83]

Can we exclude increment versioning and update changelog messages? They are internal processes that adds noise and is not needed in a Change Log.

[rjmholt]

@dragonwolf83 yes, but not in this script