Add find references

[JohnC32]

Please add the ability to find references. Having used some of the different language servers, I
find the model that C++ language server, ccls, uses to be very powerful. ccls lets one
programmatically generate an "index" for many thousands of files in a cron job. You can then
"clone/copy" this information to a new location, remap paths and use the index. Please consider
these when/if "find references" support is added:

• Ability to generate an index off-line, say in a cron job in the middle of the night.

• Ability to clone/copy the index to a new work-tree. This means having the ability to remap the "root" of
  the index file pat0hs.

• Dynamically updates as files changes. The language server should listens to file notifications (inotify).
  number of files looked at needs to be configurable.