Adding until keyword and tests (#146)

Author: StartAutomating

docs/Until.md

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+
+Until
+-----
+### Synopsis
+until keyword
+
+---
+### Description
+
+The until keyword simplifies event loops.
+
+until will always run at least once, and will run until a condition is true.
+
+---
+### Examples
+#### EXAMPLE 1
+```PowerShell
+{
+    $x = 0
+    until ($x == 10) {
+        $x            
+        $x++
+    }        
+} |.>PipeScript
+```
+
+#### EXAMPLE 2
+```PowerShell
+{
+    until "00:00:05" {
+        [DateTime]::Now
+        Start-Sleep -Milliseconds 500
+    } 
+} | .>PipeScript
+```
+
+#### EXAMPLE 3
+```PowerShell
+Invoke-PipeScript {
+    $tries = 3
+    until (-not $tries) {
+        "$tries tries left"
+        $tries--            
+    }
+}
+```
+
+---
+### Parameters
+#### **CommandAst**
+
+|Type              |Requried|Postion|PipelineInput |
+|------------------|--------|-------|--------------|
+|```[CommandAst]```|true    |named  |true (ByValue)|
+---
+### Syntax
+```PowerShell
+Until -CommandAst <CommandAst> [<CommonParameters>]
+```
+---
+### Notes
+until will become a ```do {} while ()``` statement in PowerShell.
+
+
+