Posh-hg dies in terminal without color support

[CraigStuntz]

Notably, in Powershell ISE.

1. In ISE, PS C:\Repo> Import-Module C:\Source\posh-hg\posh-hg.psm1
2. PS C:\Repo> $global:PoshHgSettings and note the colors are -1
3. PS C:\Repo> write-vcsstatus fails with 6 errors like the following:

    Write-Host : Cannot process Color because -1 is not a valid color.
    Parameter name: value
    Actual value was -1.
    At C:\Source\posh-hg\HgPrompt.ps1:16 char:19
    +         Write-Host <<<<  $s.BeforeText -NoNewline -BackgroundColor $s.BeforeBackgroundColor -ForegroundColor $s.BeforeForegroundColor
        + CategoryInfo          : InvalidArgument: (:) [Write-Host], ArgumentOutOfRangeException
        + FullyQualifiedErrorId : SetInvalidBackgroundColor,Microsoft.PowerShell.Commands.WriteHostCommand