Add Add-PoshGitToProfile -AllUsers support

[dahlbyk]

Toward including an option to install posh-git with Git for Windows (git-for-windows/git#1384), this adds an option to add to profile for all users.

Note that, as expected, -AllUsers from a non-elevated shell fails:

Add-Content : Access to the path
'C:\Windows\System32\WindowsPowerShell\v1.0\Microsoft.PowerShell_profile.ps1' is denied.

or

Add-Content : Access to the path
'C:\Windows\System32\WindowsPowerShell\v1.0\profile.ps1' is denied.

Running PowerShell as Administrator allows the global install.

[rkeithhill]

LGTM

[rkeithhill]

BTW we do have this code to determine if the process is elevated or not.

posh-git/src/GitPrompt.ps1

Lines 115 to 124 in 65658bf

	# PowerShell 5.x only runs on Windows so use .NET types to determine isAdminProcess
	# Or if we are on v6 or higher, check the $IsWindows pre-defined variable.
	if (($PSVersionTable.PSVersion.Major -le 5) -or $IsWindows) {
	$currentUser = [Security.Principal.WindowsPrincipal]([Security.Principal.WindowsIdentity]::GetCurrent())
	$isAdminProcess = $currentUser.IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)
	}
	else {
	# Must be Linux or OSX, so use the id util. Root has userid of 0.
	$isAdminProcess = 0 -eq (id -u)
	}

We could check if the process is not elevated and the user specified -AllHosts and give a better error message. One that tells them they need to be running as admin in order to update AllHosts profiles.

[dahlbyk]

We could check if the process is not elevated and the user specified -AllHosts and give a better error message.

Done.

Installing posh-git for all users requires an elevated host.

[rkeithhill]

What do you think about rewording that as 'Adding posh-git to an AllUsers profile requires an elevated host.'?

[rkeithhill]

Other than that, this PR looks good to me.