PowerShell 6 Core Support (#35)

## About
This pull request reflects all changes done in the `linuxsupport` branch.

## Content
- Enable PowerShell 6 Core support
- Use PFX Certificate for encryption ( fixes #32 )
- Updates CI / CD pipeline ( fixes #31 )
- uses portable libressl ( fixes #34 )
- adds `-PassThru` switch for returning current `VIServer` session in `Connect-To` ( fixes #34 )
- adds git lfs for embedded libressl files
- restructured internal functions into `Private` dir
- added certificate related functions
- adds travis build pipeline for tests

.vscode/settings.json

@@ -26,7 +26,8 @@
     "powershell.codeFormatting.whitespaceAroundOperator": true,
     "powershell.codeFormatting.whitespaceAfterSeparator": true,
     "powershell.codeFormatting.ignoreOneLineBlock": true,
-    "powershell.codeFormatting.alignPropertyValuePairs": false,
+    "powershell.codeFormatting.alignPropertyValuePairs": true,
+    "powershell.codeFormatting.preset": "Custom",
     // cspell spellchecker options
     "cSpell.enabledLanguageIds": [
         "c",