Implemented option for free-form text entry into combo box - see header ...

...comment notes for details.

This is my first time using github, so forgive me if this is wrong or incorrect in some manner...

A little background: I am a web developer for Fender (http://www.fender.com/) and we had a need for a combobox widget; I found your widget and it fit the need almost perfectly, but a means to enter in a freeform text entry (that wasn't in the dropdown) was also needed. A few minor changes, and what you see here is what I came up with.

I won't claim it to be totally correct, but it seems to work; if the "freeform" option is set when the widget is instantiated, my changes are used, otherwise the default is used.

I hope this makes sense; the comments in the header are just there to document this option - it would be preferable, should this change be merged into the main branch that they are stripped and become a part of the documentation of course.

Thank you.