Adding Appended and Prepended Text

Bootstrap supports appended text as well prepended text. Currently this gem only supports prepended text. I have added in a minimal amount of code to support appended text. The code is based directly off the code already in place the prepended text. I have also updated the read me as well. I have also updated it to handle appended and prepended options on a single field, where before it would only handle one or the other.

NOTE:
Previous pull request for appended text was closed as I found out that the gem could only handle appended OR prepended text. This pull fix includes the original to add appended text, but also allows appended and prepended text to be added on the same field.