support token substitution

[aerickson]

e.g. go/google/test could go to https://www.google.com/search?q=test.

The url entered into the UI would be something like:

https://www.google.com/search?q=$1

[arkarkark]

If you used %s rather than $1 then it would be the same format as mozilla bookmark keywords

http://www-archive.mozilla.org/docs/end-user/keywords.html

or perhaps openSearch is a better choice https://developer.mozilla.org/en-US/Add-ons/Creating_OpenSearch_plugins_for_Firefox then you can even support suggestions for the tokens!