Implement initial PowerShell 6.0+ tab-completions (missing help fallback, show)

extra/completion.ps1

@@ -0,0 +1,47 @@
+Class libmuttonEntries : System.Management.Automation.IValidateSetValuesGenerator {
+    [string[]] GetValidValues() {
+        $entryPath = (Resolve-Path '~/AppData/Local/libmutton/entries').Path
+        $entryNames = If (Test-Path $entryPath) {(Get-ChildItem -Path $entryPath -Recurse).FullName.Substring($entryPath.Length) -replace '\\', '/'}
+        return [string[]] $entryNames
+    }
+}
+
+function MUTNArgumentCompleter {
+    param ( $commandName,
+            $parameterName,
+            $wordToComplete,
+            $commandAst,
+            $fakeBoundParameters )
+
+$possibleValues = @{
+        add = @('password', 'note', 'folder')
+        copy = @('password', 'username', 'url', 'note')
+        edit = @('password', 'username', 'url', 'note', 'rename')
+        gen = @('update')
+    }
+
+if ($fakeBoundParameters.ContainsKey('argument')) {
+        $possibleValues[$fakeBoundParameters.argument] | Where-Object {
+            $_ -like "$wordToComplete*"
+        }
+    } else {
+        $possibleValues.Values | ForEach-Object {$_}
+    }
+}
+
+function mutn {
+[CmdletBinding()]
+ param (
+        [Parameter(Mandatory=$false, Position = 0)]
+        [ValidateSet([libmuttonEntries])]
+        [string]$entry,
+
+        [Parameter(Position = 1)]
+        [ArgumentCompletions('add', 'gen', 'edit', 'copy', 'shear')]
+        $argument,
+
+        [Parameter(Position = 2)]
+        [ArgumentCompleter({ MUTNArgumentCompleter @args })]
+        $option
+      )
+}