Feature/fuzzy finder improvements

A number of improvements to the fuzzy file finder (re: @hadley's comments)

1. Exact matches now receive no penalty (so we can be sure that the query `build` will place a function `build` at the top)

2. `json`, `yaml` and `yml` files are now indexed (although the `.yaml` mode highlighting is kind of ugly / not that useful)

3. The scores for files are penalized a bit (so that functions with equal scores are preferentially chosen)

Some other changes rolled into this one:

1. Tokenization for strings is fixed in R modes (it turns out that e.g. `"abc"` was tokenized as `['"', 'abc"']`!)
2. The scope tree in R modes now displayed the function name as well as the function arguments (I believe this will be a popular change; now one can see what arguments a function takes while they work within it)