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

src/main.rs

@@ -43,6 +43,7 @@ mod args;
 mod config;
 mod decoder;
 mod decompressor;
+mod preprocessor;
 mod logger;
 mod pathutil;
 mod printer;