Add a flag to suppress deprecation warning

Allow Powershell 2 support deprecation warning to be suppressed by
importing the module with a true value as an argument.

The ability to pass arguments through Import-Module is pretty limited,
accepting only positional arguments, but it still seems a preferable
option to environment variables or asking users to modify posh-git
directly.

Closes dahlbyk#163

posh-git.psm1

@@ -1,9 +1,12 @@
+param([switch]$NoVersionWarn = $false)
+
 if (Get-Module posh-git) { return }
 
-if ($PSVersionTable.PSVersion.Major -lt 3) {
+if ($PSVersionTable.PSVersion.Major -lt 3 -and !$NoVersionWarn) {
     Write-Warning ("posh-git support for PowerShell 2.0 is deprecated; you have version $($Host.Version).`n" +
     "To download version 3.0, please visit https://www.microsoft.com/en-us/download/details.aspx?id=34595`n" +
-    "For more information and to discuss this, please visit https://github.com/dahlbyk/posh-git/issues/163`n")
+    "For more information and to discuss this, please visit https://github.com/dahlbyk/posh-git/issues/163`n" +
+    "To suppress this warning, change your profile to include 'Import-Module posh-git -Args `$true'.")
 }
 
 Push-Location $psScriptRoot