StringPrefix and StringSuffix

[waltertamboer]

I'd like to propose two new filters which can be used to prefix or suffix a value before it is validated:

• StringPrefix - Adds a static string to the beginning of the value.
• StringSuffix - Adds a static string to the end of the value.

This can be useful in cases where an input box statically has a prefix or suffix and you want this value to be in the end-result as well. For example like this:

• http://i.imgur.com/DnlZQf2.png
• http://i.imgur.com/sSJdqpo.png

What do you think?

[thexpand]

I've created a PR for this. It has both filter with tests added to them.
@froschdesign Can you take a look at it and eventually merge it?

[weierophinney]

Fixed with #69