Tweak fuzzy scoring system

- No penalty for perfect matches (so exact matches come first)
- Functions are preferred to files (files get small penalty)
Recognize .json, .yml, some other special cases
Add more files for indexing
fix tokenization for strings
Include function signature in R scope tree
don't truncate labels
Use 'rfind' (want last '.' in string)
remove debug output; include '.' in extension
register a couple more files
unify file penalties
prefer shorter to longer matches
Merge pull request #184 from rstudio/feature/fuzzy-finder-improvements

Feature/fuzzy finder improvements