Add a make your own custom header option

I guess a many people will install this addon because they want to easily track *one* header, such as "X-Spam-Score".

This commit adds the posibility to add one truly custom column by entering the text for the header to be shown. The code could certainly be improved, but I know just about nothing about extension development and it works for me.

I haven't looked into translating the extra strings, or even including them in the language specific files.

Refs #17