Add rg-finish-functions (#142)

This would allow the same functionality as
compilation-finish-functions, i.e. add hooks called when the ripgrep
process finishes.

Could for instance be used to select the *rg* buffer after a search:
(add-to-list 'rg-finish-functions (lambda (buffer _) (pop-to-buffer buffer)))

rg-result.el

@@ -299,6 +299,12 @@ Becomes buffer local in `rg-mode' buffers.")
 (defvar rg-ellipsis (if (char-displayable-p ?…) "[…]" "[...]")
   "Used when hiding command line.")
 
+(defvar rg-finish-functions '()
+  "Functions to call when a ripgrep process finishes.
+
+Each function is called with two arguments: the compilation buffer,
+and a string describing how the process finished.")
+
 
 ;; Defuns