Add highlighting for PowerShell syntax

[rdnlsmith]

I believe I did this correctly. I followed the download link in the
previous prism.js, added the PowerShell checkbox, and followed the
steps in this comment; it seems to work. Unfortunately, the
minified JS diff is rather unreadable.

Fixes #193.

[panr]

Hi! Does this PR work for you?

Just tested it with:

```diff
-This is an old line
+This is a new line
```

and I got:

[rdnlsmith]

This PR only adds PowerShell syntax highlighting, which was the main subject of issue #193. My comment about the diff highlighting was just an aside to ask if I had done something incorrectly; if the highlighting for diffs is actually broken, perhaps I ought to split that off into a separate issue?

[panr]

This PR only adds PowerShell syntax highlighting, which was the main subject of issue #193. My comment about the diff highlighting was just an aside to ask if I had done something incorrectly;

Oh, got it 👍

if the highlighting for diffs is actually broken, perhaps I ought to split that off into a separate issue?

It would be super nice!