Add -Quiet parameter to Invoke-Pester

Fixes #223
Adds -Quiet parameter to Invoke-Pester that disables Write-Host output
to screen. Write-Screen function is added that wraps Write-Host
function. Write-Screen should be used instead of Write-Host everywhere
inside Pester to ensure all Host output is disabled when needed.
Patching the Out-Host

As @dlwyatt pointed out Out-Host was used here for a reason, to prevent
the CodeCoverage output from becoming the part of the default stream and
thus mixing with the output of the PassThru.
Replace colors with output types on Write-Screen

Improving the abstraction of the Write-Screen function to use output
types instead of colors.  That way we can easily set different color
sets for light and dark backgrounds if we choose to do that.
Merge pull request #224 from pester/Quiet_mode_for_Invoke-Pester

Quiet mode for invoke pester
Fix Write-Screen error output and color
Use Test-Path (which produces no errors) instead of Get-Variable
Update Changelog