More ISE customisation.

- Fix Set-Encoding to actually set the encoding.
- Add Close-All and Save-All (and key bindings).
- Add 'edit' alias (which makes it easy to edit specified files from the interactive window).
Changes to prompt, mostly.

- Change prompt to put PWD on a separate line -- makes it easier to copy/paste commands.
- Change title to show PWD.
- Some tidying up of the format.
- Change default vsvars to 2012.