Add icon contributions

[pseys]

The following contributions have been submitted to the Salt Design team:

Shannon Hecker

• Indent
• Outdent

Are there any other content format icons that should be included?

• Alignment e.g. left, right, centred, justified
• Text style e.g. bold, italic

Sandra Hsu

• Currency exchange

Are there any other currency-related icons that should be included?

• Euro, GBP, Dollar, Yen?

Barnali Nandan

• Excel download

Are there any other document format icons that should be included? If we're to add an Excel download icon, we need to provide the same for all other file formats offered within Salt