Update how secrets are masked.

Due to #78 and #79 it not a good idea to override the built in ps
functions in the way that we are now. I've modified how secrets are
masked. Currently secrets will always be masked. The implementation is not
quite as safe as the previous because previoulsy psbuild was overriding
commands like write-host, write-output, etc to mask secrets. That had
issues where the prompt was getting modified. Instead of that I've updated
calls that can cause output to call Get-FilteredString instead. Including
masking any exception messages. I'm pretty confident that secrets are
still being masked correctly including in log files.