libripgrep: initial commit introducing libripgrep

libripgrep is not any one library, but rather, a collection of libraries
that roughly separate the following key distinct phases in a grep
implementation:

  1. Pattern matching (e.g., by a regex engine).
  2. Searching a file using a pattern matcher.
  3. Printing results.

Ultimately, both (1) and (3) are defined by de-coupled interfaces, of
which there may be multiple implementations. Namely, (1) is satisfied by
the `Matcher` trait in the `grep-matcher` crate and (3) is satisfied by
the `Sink` trait in the `grep2` crate. The searcher (2) ties everything
together and finds results using a matcher and reports those results
using a `Sink` implementation.

Closes #162

grep-searcher/examples/search-stdin.rs

@@ -0,0 +1,33 @@
+extern crate grep_regex;
+extern crate grep_searcher;
+
+use std::env;
+use std::error::Error;
+use std::io;
+use std::process;
+
+use grep_regex::RegexMatcher;
+use grep_searcher::Searcher;
+use grep_searcher::sinks::UTF8;
+
+fn main() {
+    if let Err(err) = example() {
+        eprintln!("{}", err);
+        process::exit(1);
+    }
+}
+
+fn example() -> Result<(), Box<Error>> {
+    let pattern = match env::args().nth(1) {
+        Some(pattern) => pattern,
+        None => return Err(From::from(format!(
+            "Usage: search-stdin <pattern>"
+        ))),
+    };
+    let matcher = RegexMatcher::new(&pattern)?;
+    Searcher::new().search_reader(&matcher, io::stdin(), UTF8(|lnum, line| {
+        print!("{}:{}", lnum, line);
+        Ok(true)
+    }))?;
+    Ok(())
+}