feature request: find compile code database smarter

[wxk6b1203]

I don't think this is a good idea to bother the author of this plugin, but I think it is useful to let it search compile_command.json on a current working directory or in any directory above it in the hierarchy (recursively).

Just like ycm do so, it is more convenient to use. Such as:

$ pwd
/Users/hello/Project/hel1/regex
$ vim hello.cc
# compile_command.json will be found on /Users/hello/Project/hel1 
# instead of /Users/hello/Project/hel1/regex