Update PowerShell File Extensions

The three core PowerShell language extensions are .psd1, .ps1 and .psm1 -- plus two xml file extensions: .ps1xml and .clixml which are for formatting rules and serialization.
.psm1 modules files use exactly the same syntax as scripts, but are imported rather than executed.
.psd1 are metadata files which use a subset of the same syntax (they can be highlighted using the same highlighter, it's just some commands, variables, and types aren't allowed in data files)
Add psc1, fix order in PowerShell

I had omitted .psc1 because I wasn't confident it was xml
And I have now sorted psd1/psm1 correctly
Fix alphabetical order of ps1xml
Saying my ABCs

Fixing https://travis-ci.org/github/linguist/jobs/12531345
I swear: I am not normally dyslexic
Merge pull request #715 from Jaykul/master

Update PowerShell File Extensions