ripgrep: add --pre flag

The preprocessor flag accepts a command program and executes this
program for every input file that is searched. Instead of searching the
file directly, ripgrep will instead search the stdout contents of the
program.

Closes #978, Closes #981

complete/_rg

@@ -170,7 +170,8 @@ _rg() {
     {-w,--word-regexp}'[only show matches surrounded by word boundaries]'
     {-x,--line-regexp}'[only show matches surrounded by line boundaries]'
 
-    + '(zip)' # Compressed-file options
+    + '(input-decoding)' # Input decoding options
+    '--pre=[specify preprocessor utility]:preprocessor utility:_command_names -e'
     {-z,--search-zip}'[search in compressed files]'
     $no"--no-search-zip[don't search in compressed files]"