Added support for specific extensions to Install-ChocolateyExplorerMenuItem

One file changed.

It took me 5 commits because Git left me with duplicates in wrong folders after a pull and a push.

I ended up with:
master / chocolatey / src / helpers / functions / Install-ChocolateyExplorerMenuItem.ps1 
master / src / helpers / functions / Install-ChocolateyExplorerMenuItem.ps1 
deleted the first one.
Check out the second last commit for the actual changes.