Keyword not found recommendations

For issue #888.
I don't think removing `${}` really helps matching embedded args. Aren't they like any other character in the argument name? Most of the time the used keyword has something totally different in that place anyway.

It's a valid point that if you use `my_keyword`, you should get same recommendations as if you used `my keyword`. In the list of possible matches underscores have already been removed after using `utils.printable_name()`, though. In the given name replacing underscores with spaces, like you suggested, is better than letting `utils.normalize` to remove them. Alternatively we could revert the decision to preserve spaces in normalization. Did that have any effect to results? Removing spaces would mean that also `m y k e y w o r d` would get same recommendations as `my keyword`. I'm not sure does does removing or preserving spaces and underscores really have any effect on anything in real life.