consistent search behavior - '%s' format vs append

[faizk]

The following should result in the same search url, but they don't:

open DEFAULTENGINE needle
open? needle
open needle

If DEFAULTENGINE is configured as

let searchengine myengine = "http://my.engi.ne/search?q=%s"
let defaultengine = "myengine"

Then the 3 open commands result in:

http://my.engi.ne/search?q=needle # OK
http://my.engi.ne/search?q=%sneedle # ?
http://my.engi.ne/search?q=%sneedle # ?

This fix makes the behavior consistent regardless of whether or not the
search engine to use is explicit; from README.md -

If you leave out the '%s' at the end of the URL,
your query will be appended to the link.
Otherwise, your query will replace the '%s'.