Add a --null flag.

This flag causes a NUL byte to follow any file path in ripgrep's output.

Closes #89.

src/args.rs

@@ -155,6 +155,12 @@ Less common options:
         Don't respect version control ignore files (e.g., .gitignore).
         Note that .ignore files will continue to be respected.
 
+    --null
+        Whenever a file name is printed, follow it with a NUL byte.
+        This includes printing filenames before matches, and when printing
+        a list of matching files such as with --count, --files-with-matches
+        and --files.
+
     -p, --pretty
         Alias for --color=always --heading -n.
 
@@ -224,6 +230,7 @@ pub struct RawArgs {
     flag_no_line_number: bool,
     flag_no_mmap: bool,
     flag_no_filename: bool,
+    flag_null: bool,
     flag_pretty: bool,
     flag_quiet: bool,
     flag_regexp: Vec<String>,
@@ -269,6 +276,7 @@ pub struct Args {
     no_ignore: bool,
     no_ignore_parent: bool,
     no_ignore_vcs: bool,
+    null: bool,
     quiet: bool,
     replace: Option<Vec<u8>>,
     text: bool,
@@ -399,6 +407,7 @@ impl RawArgs {
             no_ignore_vcs:
                 // --no-ignore implies --no-ignore-vcs
                 self.flag_no_ignore_vcs || no_ignore,
+            null: self.flag_null,
             quiet: self.flag_quiet,
             replace: self.flag_replace.clone().map(|s| s.into_bytes()),
             text: text,
@@ -553,6 +562,7 @@ impl Args {
             .heading(self.heading)
             .line_per_match(self.line_per_match)
             .quiet(self.quiet)
+            .null(self.null)
             .with_filename(self.with_filename);
         if let Some(ref rep) = self.replace {
             p = p.replace(rep.clone());