Allow user to add completion for powershell alias

When a user has an alias in powershell, she will need to register that
alias for completion.  To make that possible, we store the completion
logic into a scriptblock variable which can easily be accessed by the
user to register aliases.

For example, if the user defines an alias for `helm`:
   PS> sal h helm
she will need to register the alias like so:
   PS> Register-ArgumentCompleter -CommandName 'h' -ScriptBlock $__helmCompleterBlock

Merge spf13/cobra#1621

site/content/completions/powershell.md

@@ -36,6 +36,20 @@ $ helm s[tab]
 search  show  status
 ```
 
+### Aliases
+
+You can also configure `powershell` aliases for your program, and they will also support completions.
+
+```
+$ sal aliasname origcommand
+$ Register-ArgumentCompleter -CommandName 'aliasname' -ScriptBlock $__origcommandCompleterBlock
+# and now when you run `aliasname` completion will make
+# suggestions as it did for `origcommand`.
+$ aliasname <tab>
+completion     firstcommand   secondcommand
+```
+The name of the completer block variable is of the form `$__<programName>CompleterBlock` where every `-` and `:` in the program name have been replaced with `_`, to respect powershell naming syntax.
+
 ### Limitations
 
 * The following flag completion annotations are not supported and will be ignored for `powershell`: