Add PowerShell syntax test

Part of the work for issue sharkdp#1213.

A short PowerShell script and the corresponding syntax-highlighted output
has been added. The script exhibits many of the peculiarities of the
PowerShell syntax.

tests/syntax-tests/highlighted/PowerShell/test.ps1

@@ -0,0 +1,25 @@
+# PowerShell script for testing syntax highlighting
+
+function Get-FutureTime {
+    param (
+        [Int32] $Minutes
+    )
+    
+    $time = Get-Date | % { $_.AddMinutes($Minutes) }
+    "{0:d2}:{1:d2}:{2:d2}" -f @($time.hour, $time.minute, $time.second)
+}
+
+if ($env:PATH -match '.*rust.*') {
+   'Path contains Rust'
+    try {
+        & "cargo" "--version"
+    } catch {
+        Write-Error "Failed to run cargo"
+    }
+} else {
+   'Path does not contain Rust'
+}
+
+(5..30) | ? { $_ % (2 * 2 + 1) -eq 0 } | % {"In {0} minutes, the time will be {1}." -f $_, $( Get-FutureTime $_ )}
+$later =  Get-FutureTime -Minutes $( Get-Random -Minimum 60 -Maximum 120 )
+"The time will be " + $later + " later."