ripgrep: migrate to libripgrep

This commit does the work to delete the old `grep` crate and effectively
rewrite most of ripgrep core to use the new libripgrep crates. The new
`grep` crate is now a facade that collects the various crates that make
up libripgrep.

The most complex part of ripgrep core is now arguably the translation
between command line parameters and the library options, which is
ultimately where we want to be.

grep/README.md

@@ -1,4 +1,41 @@
 grep
 ----
-This is a *library* that provides grep-style line-by-line regex searching (with
-comparable performance to `grep` itself).
+ripgrep, as a library.
+
+[![Linux build status](https://api.travis-ci.org/BurntSushi/ripgrep.svg)](https://travis-ci.org/BurntSushi/ripgrep)
+[![Windows build status](https://ci.appveyor.com/api/projects/status/github/BurntSushi/ripgrep?svg=true)](https://ci.appveyor.com/project/BurntSushi/ripgrep)
+[![](https://img.shields.io/crates/v/grep.svg)](https://crates.io/crates/grep)
+
+Dual-licensed under MIT or the [UNLICENSE](http://unlicense.org).
+
+
+### Documentation
+
+[https://docs.rs/grep](https://docs.rs/grep)
+
+NOTE: This crate isn't ready for wide use yet. Ambitious individuals can
+probably piece together the parts, but there is no high level documentation
+describing how all of the pieces fit together.
+
+
+### Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+grep = "0.2"
+```
+
+and this to your crate root:
+
+```rust
+extern crate grep;
+```
+
+
+### Features
+
+This crate provides a `pcre2` feature (disabled by default) which, when
+enabled, re-exports the `grep-pcre2` crate as an alternative `Matcher`
+implementation to the standard `grep-regex` implementation.