output: add --stats flag

This commit provides basic support for a --stats flag, which will print
various aggregate statistics about a search after all of the results
have been printed. This is mostly intended to support a similar feature
found in the Silver Searcher. Note though that we don't emit the total
bytes searched; this is a first pass at an implementation and we can
improve upon it later.

Closes #411, Closes #799

complete/_rg

@@ -91,6 +91,7 @@ _rg() {
     '(-e -f --file --files --regexp --type-list)1: :_rg_pattern'
     '(--type-list)*:file:_files'
     '(-z --search-zip)'{-z,--search-zip}'[search in compressed files]'
+    "(--stats)--stats[print stats about this search]"
   )
 
   [[ ${_RG_COMPLETE_LIST_ARGS:-} == (1|t*|y*) ]] && {