Replace `powershell/encode` encoder with a `powershell/hex_encode` encoder

[postmodern]

The powershell/encode encoder does not actually do anything. It should be replaced with a powershell/hex_encode encoder which hex encodes each character of the PowerShell command string and evaluates it using Invoke-Expression.

dir -> Invoke-Expression "$([char]0x64)$([char]0x69)$([char]0x72)"

[postmodern]

Implemented by 95f6726.