Allow to search tags table first

Add new user option elisp-slime-nav-try-tags-first.  When non-nil, try to find the symbol with find-tag first.  For compatibility, this option defaults to nil.

Still lacks doc in README, but I'm opening the PR anyway to get your feedback first.


When working on Flycheck, I want to jump to the definition as present in the current source code tree.  Without this option, however, elisp-slime-nav jumps to the definition from the Flycheck package I installed for daily use.

To make elisp-slime-nav use the tags from the Flycheck source code, I have to eval the Flycheck buffer first, which I often forget, and often don't even want to, for instance, if the Flycheck source code is currently in a broken state, because I haven't finished implementing a new feature yet.

With this option however, I can just generate a tag table for Flycheck (I use `projectile-regenerate-tags` for this purpose), and use elisp-slime-nav to navigate my Flycheck source code.  But for symbols from other libraries, elisp-slime-nav will still jump to the current definition.